Add readme for podspec presubmit tests (#7968)

* Add readme for podspec presubmit tests

Author: granluo

scripts/create_spec_repo/README.md

@@ -0,0 +1,48 @@
+# Podspec presubmit test setup
+
+Podspec presubmit test is to help ensure podspec is releasable. 'pod spec lint' will run for SDKs
+with sources of
+
+- https://github.com/firebase/SpecsTesting
+- https://github.com/firebase/SpecsDev.git
+- https://github.com/firebase/SpecsStaging.git
+- https://cdn.cocoapods.org/
+
+where [SpecsTesting](https://github.com/firebase/SpecsTesting) is generated from the head of the
+master branch of [firebase-ios-sdk repo](https://github.com/firebase/firebase-ios-sdk).
+
+The [prerelease workflow](https://github.com/firebase/firebase-ios-sdk/blob/master/.github/workflows/prerelease.yml#L11-L46)
+will update the [SpecsTesting repo](https://github.com/firebase/SpecsTesting) nightly from the
+head of the master branch.
+In order to let presubmit tests run on the latest podspec repo, [SpecsTesting repo](https://github.com/firebase/SpecsTesting)
+will be updated when a PR with changed podspecs is merged.
+When this PR is merged, changed podspecs will be `pod repo push`ed to the podspec repo in
+[postsubmit tests](https://github.com/firebase/firebase-ios-sdk/blob/master/.github/workflows/prerelease.yml#L48-L94).
+
+Since `pod spec lint` will test podspecs with remote sources. One PR with changes on multiple
+podspecs are not encouraged. Changes with multiple podspecs, including their dependencies, might
+fail presubmit tests.
+
+## Set up presubmit tests
+
+To set up presubmit tests, we can add a new job in SDK workflows. An example of `FirebaseDatabase`
+is shown below.
+`github.event.pull_request.merged != true && github.event.action != 'closed'` is to trigger this
+job in presubmit.
+```
+  podspec-presubmit:
+    # Don't run on private repo unless it is a PR.
+    if: github.repository == 'Firebase/firebase-ios-sdk' && github.event.pull_request.merged != true && github.event.action != 'closed'
+    runs-on: macOS-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup Bundler
+      run: scripts/setup_bundler.sh
+    - name: Build and test
+      run: scripts/third_party/travis/retry.sh pod spec lint FirebaseDatabase.podspec --skip-tests --sources='https://github.com/firebase/SpecsTesting','https://github.com/firebase/SpecsDev.git','https://github.com/firebase/SpecsStaging.git','https://cdn.cocoapods.org/'
+
+```
+
+Once a PR is merged, [`update_SpecsTesting_repo` job](https://github.com/firebase/firebase-ios-sdk/blob/master/.github/workflows/prerelease.yml#L48)
+in the [prerelease workflow](https://github.com/firebase/firebase-ios-sdk/blob/master/.github/workflows/prerelease.yml)
+will automatically `pod repo push` changed podspecs in postsubmits,