Add pull request labeler (JuliaPlots#301)

This adds a pull request labeler using
[Github Actions](https://github.com/actions/labeler).

Author: abhishalya

.github/labeler.yml

@@ -0,0 +1,6 @@
+documentation:
+- "docs/**/*"
+CI:
+- "**/*.yml"
+testing:
+- test/*

.github/workflows/label.yml

@@ -0,0 +1,11 @@
+name: "Labeler"
+on:
+- pull_request
+
+jobs:
+  triage:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/labeler@v2
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"