house-keeping: PR template with submission checklist

also check PR source branch (head_ref) is not main/master

.github/pull_request_template.md

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+<!-- 
+
+Thank you for your contribution to the luci repository.
+
+Please read this before creating your PR.
+
+Review https://github.com/openwrt/luci/blob/master/CONTRIBUTING.md
+especially if this is your first time to contribute to this repo.
+
+MUST NOT:
+- add a PR from your *main* branch - put it on a separate branch
+- add merge commits to your PR: rebase locally and force-push
+
+MUST:
+- increment any PKG_VERSION in the affected Makefile
+- set to draft if this PR depends on other PRs to e.g. openwrt/openwrt
+- each commit subject line starts with '<package name>: title' 
+- each commit has a valid `Signed-off-by: ` (S.O.B.) with a reachable email
+	* Forgot? `git commit --amend ; git push -f`
+	* Tip: use `git commit --signoff`
+
+MAY:
+- your S.O.B. *may* be a nickname
+- delete the below *optional* entries that do not apply
+- skip a `<package name>: title` first line subject if the commit is house-keeping or chore
+
+-->
+
+- [ ] This PR is not from my *main* or *master* branch :poop:, but a *separate* branch :white_check_mark:
+- [ ] Each commit has a valid :black_nib: `Signed-off-by: <my@email.address>` row (via `git commit --signoff`)
+- [ ] Each commit and PR title has a valid :memo: `<package name>: title` first line subject for packages
+- [ ] Incremented :up: any `PKG_VERSION` in the Makefile
+- [ ] Tested on: (architecture, openwrt version, browser) :white_check_mark:
+- [ ] \( Preferred ) Mention: @ the original code author for feedback
+- [ ] \( Preferred ) Screenshot or mp4 of changes:
+- [ ] \( Optional ) Closes: e.g. openwrt/luci#issue-number
+- [ ] \( Optional ) Depends on: e.g. openwrt/packages#pr-number in sister repo
+- [ ] Description: (describe the changes proposed in this PR)

.github/workflows/check_source_branch_is_not_master.yml

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+name: Check PR source branch
+
+on:
+  pull_request_target:
+    types: [opened, synchronize]
+
+jobs:
+  check-source-branch-is-not-main-or-master:
+    runs-on: ubuntu-latest
+    if:  contains(fromJSON('["main", "master"]'), github.head_ref)
+    # or github.event.pull_request.head.ref
+    steps:
+      - run: |
+          echo "Error: Pull requests should not be from your 'main' or 'master' branch. :("
+          exit 1