Add release procedure notes

RELEASING.md

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+# Making a rustls-native-certs release
+
+This is a checklist for steps to make before/after making a rustls release.
+
+1. Attend to the README.md: this appears on crates.io for the release, and can't be edited after
+   the fact.
+   - Ensure the version has a good set of release notes.  Move old release notes to OLDCHANGES.md
+     if this is getting excessively long.
+   - Write the version and date of the release.
+2. Run `cargo update` followed by `cargo outdated`, to check if we have any
+   dependency updates which are not already automatically taken by their semver specs.
+   - If we do, take them if possible with separate commits (but there should've been
+     dependabot PRs submitted for these already.)
+3. Now run `cargo test --all-features` to ensure our tests continue to pass with the
+   updated dependencies.
+4. Update `Cargo.toml` to set the correct version.
+5. Make a commit with the above changes, something like 'Prepare $VERSION'.  This
+   should not contain functional changes: just versions numbers, and markdown changes.
+6. Do a dry run: check `cargo publish --dry-run`
+7. Push the above commit.  Wait for CI to confirm it as green.
+   - Any red _should_ naturally block the release.
+   - If rustc nightly is broken, this _may_ be acceptable if the reason is understood
+     and does not point to a defect.
+8. Tag the released version: `git tag -m '0.20.0' v/0.20.0`
+9. Push the tag: `git push --tags`
+10. Do the release: `cargo publish`.