More CI updates (google#808)

- Run test builds on push to master and nightly.
- Update the badge link to point to the actions page
- Use Github's generated badge markdown.

.github/workflows/ci.yml

@@ -1,7 +1,19 @@
-on: [pull_request]
+on:
+  pull_request:
+  # Run on all pull requests.
+
+  push:
+    # Run on merges/pushes to master.
+    branches:
+      - master
+
+  schedule:
+    # Run nightly, at midnight.
+    - cron: '8 0 * * *'
+
 name: draco-ci
-jobs:
 
+jobs:
   # Main build and test job.
   draco-tests:
     strategy:

README.md

@@ -2,8 +2,7 @@
 <img width="350px" src="docs/artwork/draco3d-vert.svg" />
 </p>
 
-![draco-ci](https://github.com/google/draco/actions/workflows/ci.yml/badge.svg)
-
+[![draco-ci](https://github.com/google/draco/actions/workflows/ci.yml/badge.svg)](https://github.com/google/draco/actions/workflows/ci.yml)
 
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