[ci] Don't kick off CI for documentation only changes. (dotnet#712)

Use the [YAML `paths:` feature][0] of CI triggers to prevent running
CI for PRs that only contain "Documentation" changes.

Note: I wasn't able to verify that this works.  It's possible that it
needs to already be on the default branch in order to take effect.
It shouldn't hurt to merge it and then test.

[0]: https://docs.microsoft.com/en-us/azure/devops/pipelines/repos/github?view=azure-devops&tabs=yaml#paths

Author: jpobst

build-tools/automation/azure-pipelines.yaml

@@ -5,8 +5,14 @@ trigger:
   - d16-*
 
 pr:
-  - master
-  - d16-*
+  branches:
+    include:
+    - master
+    - d16-*
+  paths:
+    exclude:
+    - README.md
+    - Documentation/*
 
 # Global variables
 variables: