Add some docs about how to release new versions

contrib/how_to_release.md

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+# How to release new versions of the Ruma crates
+
+## General
+
+* When releasing a pre-release version, make sure other crates are updated to
+  depend on it, with an exact version requirement (`=x.y.z-tag`)
+* When releasing a regular version, make sure other crates are updated to depend
+  on it, with a regular (same as caret / `^`) version requirement (`x.y.z`)
+* Macro crates are versioned identically to their "parent" crate and are exempt
+  from the rule above: Dependencies from the parent crate to the macro crate
+  (for re-exporting) should always use exact version requirements. Whenever a
+  crate with an associated macro crate should get a new release, release a new
+  version of the macro crate with the same number first, even if there were no
+  changes in the macro code.
+
+## Dependencies
+
+Dependencies obviously need to be released before dependents. Also, a breaking
+change release in a dependency should usually be followed by a new release of
+all dependents.
+
+![crate dependencies](./workspace_deps.png)
+
+<small><code>cargo depgraph --all-features --exclude syn,quote,js_int,trybuild,criterion,proc-macro2,serde,http,form_urlencoded,serde_json,proc-macro-crate,rand,either,ring,base64,itoa,untrusted,futures-util,hyper,hyper-tls,hyper-rustls,thiserror,paste,assign,futures-core,maplit,percent-encoding --dedup-transitive-deps | dot -Tpng</code></small>