add a [skip ci] conditional

Author: ctoney

.github/workflows/R-CMD-check.yaml

@@ -2,7 +2,7 @@
 # Need help debugging build failures? Start at https://github.com/r-lib/actions#where-to-find-help
 on:
   push:
-    branches: [main, master]
+    branches: [main]
   pull_request:
 
 name: R-CMD-check
@@ -11,6 +11,8 @@ permissions: read-all
 
 jobs:
   R-CMD-check:
+    if: "!contains(github.event.head_commit.message, '[skip ci]')"
+
     runs-on: ${{ matrix.config.os }}
 
     name: ${{ matrix.config.os }} (${{ matrix.config.r }})