Add contributor guide

CONTRIBUTING.md

@@ -0,0 +1,172 @@
+Welcome! Thanks for looking into contributing to our project!
+
+# Table of Contents
+
+- [Looking for Help?](#looking-for-help)
+  - [Documentation](#documentation)
+  - [Chat Rooms](#chat-rooms)
+- [Reporting Issues](#reporting-issues)
+- [Submitting Code](#submitting-code)
+  - [Coding Style](#coding-style)
+  - [Modifying Endpoints](#modifying-endpoints)
+  - [Submitting PRs](#submitting-prs)
+  - [Where do I start?](#where-do-i-start)
+- [Testing](#testing)
+- [Contact](#contact)
+
+# Looking for Help?
+
+Here is a list of helpful resources you can consult:
+
+## Documentation
+
+- [Matrix spec Documentation](https://matrix.org/docs/spec/client_server/latest)
+- Documentation to other Ruma modules:
+  - [ruma-events](https://docs.rs/ruma-events/)
+  - [ruma-api](https://docs.rs/ruma-api/)
+  - [ruma-client](https://docs.rs/ruma-client/)
+
+## Chat Rooms
+
+- Ruma Matrix room: [#ruma:matrix.org](https://matrix.to/#/#ruma:matrix.org)
+- Matrix Developer room: [#matrix-dev:matrix.org](https://matrix.to/#/#matrix-dev:matrix.org)
+
+# Reporting Issues
+
+If you find any bugs, inconsistencies or other problems, feel free to submit
+a GitHub [issue](issues).
+
+If you have a quick question, it may be easier to leave a message on
+[#ruma:matrix.org](https://matrix.to/#/#ruma:matrix.org).
+
+Also, if you have trouble getting on board, let us know so we can help future
+contributors to the project overcome that hurdle too.
+
+# Submitting Code
+
+Ready to write some code? Great! Here are some guidelines to follow to
+help you on your way:
+
+## Coding Style
+
+### Import Formatting
+
+Organize your imports into three groups separated by blank lines:
+
+1. `std` imports
+1. External imports (from other crates)
+1. Local imports (`self::`, `super::`, `crate::` and things like `LocalType::*`)
+
+For example,
+
+```rust
+use std::collections::HashMap;
+
+use ruma_api::ruma_api;
+
+use super::MyType;
+```
+
+Also, group imports by module. For example, do this:
+
+```rust
+use std::{
+    collections::HashMap,
+    convert::TryFrom,
+    fmt::{Debug, Display, Error as FmtError, Formatter},
+};
+```
+
+as opposed to:
+
+```rust
+use std::collections::HashMap;
+use std::convert::TryFrom;
+use std::fmt::{Debug, Display, Error as FmtError, Formatter};
+```
+
+### Code Formatting and Linting
+
+Use `rustfmt` to format your code and `clippy` to lint your code. Before
+committing your changes, go ahead and run `cargo fmt` and `cargo clippy
+--all-targets --all-features` on the repository to make sure that the
+formatting and linting checks pass in CI. Note that `clippy` warnings are
+reported as errors in CI builds, so make sure to handle those before
+comitting as well. (To install the tools, run `rustup component add rustfmt
+clippy`.)
+
+### Commit Messages
+
+Write commit messages using the imperative mood, as if completing the sentence:
+"If applied, this commit will \_\_\_." For example, use "Fix some bug" instead
+of "Fixed some bug" or "Add a feature" instead of "Added a feature".
+
+(Take a look at this
+[blog post](https://www.freecodecamp.org/news/writing-good-commit-messages-a-practical-guide/)
+for more information on writing good commit messages.)
+
+## Modifying Endpoints
+
+### Matrix Spec Version
+
+Use the latest r0.x.x documentation when adding or modifying code. We target
+the latest minor version of the Matrix specification. (Note: We might
+reconsider this when the Client-Server API hits r1.0.0.)
+
+### Endpoint Documentation Header
+
+Add a comment to the top of each endpoint file that includes the path
+and a link to the documentation of the spec. You can use the latest
+version at the time of the commit. For example:
+
+```rust
+//! [GET /.well-known/matrix/client](https://matrix.org/docs/spec/client_server/r0.4.0#get-well-known-matrix-client)
+```
+
+### Naming Endpoints
+
+When adding new endpoints, select the module that fits the purpose of the
+endpoint. When naming the endpoint itself, you can use the following
+guidelines:
+- The name should be a verb describing what the client is requesting, e.g.
+  `get_some_resource`.
+- Endpoints which are basic CRUD operations should use the prefixes
+  `create`, `get`, `update`, and `delete`.
+- The prefix `set` is preferred to create if the resource is a singleton.
+  In other words, when there's no distinction between `create` and `update`.
+- Try to use names that are as descriptive as possible and distinct from
+  other endpoints in all other modules. (For example, instead of
+  `r0::room::get_event`, use `r0::room::get_room_event`).
+- If you're not sure what to name it, pick any name and we can help you
+  with it.
+
+### Tracking Changes
+
+Add your changes to the [change log](CHANGELOG.md). If possible, try to
+find and denote the version of the spec that included the change you are
+making.
+
+## Submitting PRs
+
+Once you're ready to submit your code, create a pull request, and one of our
+maintainers will review it. Once your PR has passed review, a maintainer will
+merge the request and you're done! 🎉
+
+## Where do I start?
+
+If this is your first contribution to the project, we recommend taking a look
+at one of the [open issues][] we've marked for new contributors.
+
+It may be helpful to peruse some of the documentation for `ruma-events` and
+`ruma-api` listed above for some context.
+
+[open issues]: https://github.com/ruma/ruma-client-api/issues?q=is%3Aopen+is%3Aissue+label%3Aeffort%2Feasy
+
+# Testing
+
+Before committing, run `cargo check` to make sure that your changes can build, as well as running the formatting and linting tools [mentioned above](#code-formatting-and-linting).
+
+# Contact
+
+Thanks again for being a contributor! If you have any questions, join us at
+[#ruma:matrix.org](https://matrix.to/#/#ruma:matrix.org).