Document and codify the release process

The `wasmtime` release procees seems like it's been a bit ad-hoc up to
this point, so I figured it'd be good to try to document what we do
today and codify what should be done as well as a form of release
checklist.

I've noticed that we have a number of releases (like v0.11.0) but the
`Cargo.toml` files in the repository don't reflect the current version
of `wasmtime`. Additionally I've noticed that the [most recent release]
ended up having failed tests because `Cargo.toml` was modified but
`Cargo.lock` wasn't updated. I'm hoping that by having a checklist we
can avoid these sorts of accidental issues in the future!

[release]: https://github.com/bytecodealliance/wasmtime/runs/434690272

docs/SUMMARY.md

@@ -33,5 +33,6 @@
   - [Testing](./contributing-testing.md)
   - [Fuzzing](./contributing-fuzzing.md)
   - [CI](./contributing-ci.md)
+  - [Release Process](./contributing-release-process.md)
   - [Governance](./contributing-governance.md)
   - [Code of Conduct](./contributing-coc.md)