Improve docs of CI workflow

.github/workflows/ci.yml

@@ -1,12 +1,12 @@
 name: CI
 
 on:
-  pull_request:
+  pull_request: # Need to run on pull-requests, otherwise PRs from forks don't run
   push:
     branches:
-      - 'staging'
-      - 'trying'
-      - 'master'
+      - 'staging' # Bors uses this branch
+      - 'trying' # Bors uses this branch
+      - 'master' # Always build head of master for the badge in the README
 
 jobs:
   static_analysis: