Merge branch 'adding-bb-test-doc' into 'master'

adding the test-package-against-bb doc

See merge request !3992

docs/developer/test-package-against-bb.md

+# Testing your package branch against bigbang before package merge
+
+These instructions right now are written for istio changes, but the same is probably true for kyverno and maybe for others as well. CODEOWNERS reviewing MRs should enforce this.
+
+## Run bigbang tests against your branch
+As part of your MR that modifies istio you will need to run bigbang tests against your branch. To do this, at a minimum, you will need to
+1. Create a new branch on bigbang off of master `git checkout master && git pull && git checkout -b my-bigbang-branch-for-testing`
+1. Modify the test values (you may need more than this)
+    ```yaml
+    myAppPackage:
+      git:
+        tag: null
+        branch: my-package-branch-that-needs-testing
+      istio:
+        hardened:
+          enabled: true
+    ```
+1. Stage your changes `git add -A`
+1. Commit your changes `git commit -m "prepping for test"`
+1. Push your changes `git push -u origin my-bigbang-branch-for-testing`
+1. Create the bigbang MR as a draft with `TEST ONLY DO NOT MERGE` in the title
+1. Wait for tests to finish, and do fixes on your package branch as needed until they pass
+1. Close the bigbang MR by deleting the bigbang branch `git push -d origin my-bigbang-branch-for-testing`
+1. Link the bigbang MR on your package MR as evidence of your package working in bigbang