Add publish instructions (#395)

Author: benjie

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+## Publishing
+
+This package uses a custom publish flow.
+
+**IMPORTANT**: Do not push directly to `main` - every change must go through a PR
+otherwise changelog generation will fail.
+
+```sh
+npm install
+npm version patch # or minor or major
+git push --follow-tags
+npm run build
+cd npmDist
+npm publish --tag=next
+```
+
+Then test it by installing `graphql-relay@next` from npm...
+
+All good? Publish:
+
+```sh
+npm dist-tags add graphql-relay@VERSION_NUMBER latest
+```
+
+Finally generate the CHANGELOG:
+
+```sh
+node resources/gen-changelog.js
+```