Run CI workflow on push to #main & manual request

We used to always run the CI workflow on push/merge to #main and at some point stopped; 
we still link to the status of this workflow on #main from our README. I think we should bring it back.
Also allows manual triggering of the workflow, which can come in handy if you are working
on a branch but haven't opened a PR yet.

.github/workflows/ci.yml

@@ -1,6 +1,11 @@
 name: CI
 
-on: pull_request
+on:
+  workflow_dispatch: 
+  pull_request:
+  push:
+    branches:
+      - main
 
 jobs: