Adding some release documentation for my own reference

docs/RELEASE.md

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+These are notes, mainly to myself, for making a release.
+
+ * The versioning scheme is `my major.my minor.ffmpeg version`. So, e.g., my own
+   3.1 with FFmpeg 4.1.5 is `3.1.4.1.5`. NPM only supports three parts, so that
+   becomes `3.1.4`.
+
+ * Before making a release, run the tests with coverage. I don't presently try
+   to make sure that the tests actually have perfect coverage, but it's at least
+   a good way to know what I'm missing.
+
+ * Check library versions. Anything but FFmpeg is usually trivial to update, so
+   can be updated shortly before release.
+
+ * Run `tools/fragment-sizes.sh`.
+
+ * When updating the version, follow the instructions in package.json.
+
+ * Make a release with `make release`, then publish it to NPM with `make
+   npm-publish`. Publish the tarball to github.