Document release process

RELEASING.md

@@ -0,0 +1,48 @@
+How to Perform a Release
+========================
+
+1. The first step is to create a tag, push it, and let GitHub Actions do the release.
+
+    ```sh
+    # For v1.x.y
+    ver=1.2.0 # change this to new version
+    git checkout series/1.x
+    git checkout -b topic/v$ver
+    echo "ThisBuild / version := \"$ver\" // Workaround for #504" > version.sbt
+    git add version.sbt
+    git commit -m "Setting version to v$ver"
+    git tag -s v$ver
+    git push --tags
+
+    # For v2.x onwards
+    ver=2.0.0 # change this to new version
+    git checkout master
+    git tag -s v$ver
+    git push --tags
+    ```
+
+
+2. Wait for the release to complete.
+   https://github.com/scala-js/scala-js-dom/actions/workflows/release.yml
+
+3. Go to GitHub releases (https://github.com/scala-js/scala-js-dom/releases),
+   edit the auto-generated draft release,
+   revise,
+   and click Publish.
+
+4. Advertise the new release in the following places:
+
+    * Reddit
+      Example: https://old.reddit.com/r/scala/comments/pc9k5y/scalajsdom_120_released_this_is_the_first_release
+
+    * Scala.js Discord
+      Example: https://discord.com/channels/632150470000902164/635668814956068864/880575154516819978
+
+    * Scala.js gitter
+      Example: https://gitter.im/scala-js/scala-js?at=612811ec1179346966e36def
+
+    * scala-users
+      Example: https://users.scala-lang.org/t/scala-js-dom-v1-2-0-released/7745
+
+    * Twitter
+      Example: https://twitter.com/japgolly/status/1431011200771379200?s=20