runs test on main branch as well. (#1429)

* runs test on main branch as well.

reasons:
- tests passed on PR branches do not mean they will pass on main branch,
  for e.g. alice changed A in branch-a, bob changed B in branch-b, both
  tests passed and no conflict, but after merged to main, a bug occurs.
- wanna fix the badge in main branch ;D

* change the badge status to main branch only.

.github/workflows/ci.yml

@@ -1,6 +1,9 @@
 name: pgcli
 
 on:
+  push:
+    branches:
+      - main
   pull_request:
     paths-ignore:
       - '**.rst'

README.rst

@@ -373,8 +373,8 @@ interface to Postgres database.
 Thanks to all the beta testers and contributors for your time and patience. :)
 
 
-.. |Build Status| image:: https://github.com/dbcli/pgcli/workflows/pgcli/badge.svg
-    :target: https://github.com/dbcli/pgcli/actions?query=workflow%3Apgcli
+.. |Build Status| image:: https://github.com/dbcli/pgcli/actions/workflows/ci.yml/badge.svg?branch=main
+    :target: https://github.com/dbcli/pgcli/actions/workflows/ci.yml
 
 .. |CodeCov| image:: https://codecov.io/gh/dbcli/pgcli/branch/master/graph/badge.svg
    :target: https://codecov.io/gh/dbcli/pgcli