Add a contributing.md and explain how to do publishes. (#237)

contributing.md

@@ -0,0 +1,40 @@
+# Contributing
+
+## Making Pull Requests
+
+When making a pull request, add a changeset which helps with releases.
+
+```shell
+yarn changeset
+```
+
+This will prompt you asking what type of change was made.
+
+## Releases
+
+For those contributors that have access to publish to npm, the following is how you do releases.
+
+From the `main` branch do a pull, install and build:
+
+```shell
+git pull origin main
+yarn install
+yarn build
+```
+
+Then bump the versions:
+
+```shell
+yarn changeset version
+```
+
+This should change package.json bumping version numbers and update changelogs. Inspect these to make sure they are what you expect.
+
+Commit and push these changes, then run an npm publish for each of the packages that have changed.
+
+> __Important__! Ordering publishes can be important. If `astro-parser` changes you should publish that before `astro`, for example.
+
+```shell
+cd packages/astro
+npm publish
+```