Include more detailed Headless Testing section (#81)

I tried to get a build working yesterday via Github Actions and kept hitting a wall. Found these suggestions on `browser-run` and copied here to make it easier to find.

Author: manzt

README.md

@@ -160,9 +160,33 @@ $ browserify test.js | tape-run --render="tap-spec"
 
 ```
 
-## Headless testing / Travis
+## Headless testing
 
-To use tape-run with travis, we recommend using the default electron browser, which however requires you to add this part to your travis.yml:
+In environments without a screen, you can use `Xvfb` to simulate one. We recommend using the default electron browser, 
+which however requires you to add additional parts to your headless configurations.
+
+### GitHub Actions
+
+This is a full example to run `npm test`. Refer to the last 2 lines in the YAML config:
+
+```yml
+on:
+  - pull_request
+  - push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - run: npm install
+    - run: sudo apt-get install xvfb
+    - run: xvfb-run --auto-servernum npm test
+```
+
+### Travis
+
+Add this to your travis.yml:
 
 ```yml
 addons:
@@ -175,9 +199,9 @@ install:
   - npm install
 ```
 
-[Source](https://github.com/rhysd/Shiba/blob/055a11a0a2b4f727577fe61371a88d8db9277de5/.travis.yml).
+[Full example](https://github.com/rhysd/Shiba/blob/055a11a0a2b4f727577fe61371a88d8db9277de5/.travis.yml).
 
-For gnu/linux installations without a graphical environment:
+### Any gnu/linux box
 
 ```bash
 $ sudo apt-get install xvfb # or equivalent
@@ -186,7 +210,9 @@ $ Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
 $ browser-run ...
 ```
 
-There is also an example docker machine [here](https://github.com/fraserxu/docker-tape-run).
+### Docker
+
+There is also an example [Docker image](https://hub.docker.com/r/kipparker/docker-tape-run). [Source](https://github.com/fraserxu/docker-tape-run)
 
 ## Installation