Sync eng/common directory with azure-sdk-tools repository (#7153)

eng/common/README.md

@@ -0,0 +1,12 @@
+# Common Engineering System
+
+The `eng/common` directory contains engineering files that are common across the various azure-sdk language repos.
+It should remain relatively small and only contain textual based files like scripts, configs, or templates. It
+should not contain binary files as they don't play well with git.
+
+# Updating
+
+Any updates to files in the `eng/common` directory should be made in the [azure-sdk-tools](https://github.com/azure/azure-sdk-tools) repo.
+All changes made will cause a PR to created in all subscribed azure-sdk language repos which will blindly replace all contents of
+the `eng/common` directory in that repo. For that reason do **NOT** make changes to files in this directory in the individual azure-sdk
+languages repos as they will be overwritten the next time an update is taken from the common azure-sdk-tools repo.