Add workflow_dispatch to Create Release Pull Request workflow

Add the `workflow_dispatch` trigger to the Create Release Pull Request
workflow. This will allow us to more easily trigger the release Pull
Request if either 1) the scheduled run failed for whatever reason or 2)
an extra release is needed as per the DISCLAIMER.md.

You can read more about `workflow_dispatch` in the GitHub docs at:
https://docs.github.com/en/actions/reference/events-that-trigger-workflows#workflow_dispatch

.github/workflows/create-release.yml

@@ -1,9 +1,18 @@
 name: Create Release Pull Request
 on:
+  workflow_dispatch:
   schedule:
     # "At 00:00 on Sunday" (https://crontab.guru/once-a-week)
     - cron: "0 0 * * 0"
 
+# This Workflow can be triggered manually through the GitHub UI or API. For the
+# API use the following request:
+#   curl -X POST \
+#        -H "Authorization: Bearer <token>" \
+#        -d '{"ref":"develop"}' \
+#        https://api.github.com/repos/simple-icons/simple-icons/actions/workflows/create-release.yml/dispatches
+# Replacing <token> by a personal access token with scope `public_repo`
+
 jobs:
   release-pr:
     runs-on: ubuntu-latest