Add a description for installing GraphicsMagick or ImageMagick in doc…

…s/README.md (#4045)

* Add a troubleshooting in README for an error about gm

* Solve linting

* Change dirname from local to anonymous in the example error message

* Add mentioning ImageMagick

docs/README.md

@@ -19,6 +19,21 @@ _So you wanna build the site?_
 - The content lives in `docs/index.md`; everything else is markup, scripts, assets, etc.
 - This file (`docs/README.md`) should _not_ be included in the build.
 - `docs/_dist` is where the deployed site lives. `docs/_site` is essentially a build step. These directories are _not_ under version control.
+- If you get the error message below when running `npm start docs`, follow [this guide](https://www.npmjs.com/package/gm) to install GraphicsMagick or ImageMagick.
+
+```console
+⚠ WARN: docs/_site/images/matomo-logo.png: Error executing Stream: The gm stream ended without emitting any data
+(node:45255) UnhandledPromiseRejectionWarning: Error: docs/_site/images/matomo-logo.png: Error executing Stream: The gm stream ended without emitting any data
+    at Socket.stdout.on.once (/Users/username/mocha/node_modules/express-processimage/lib/getFilterInfosAndTargetContentTypeFromQueryString.js:821:31)
+    at Object.onceWrapper (events.js:286:20)
+    at Socket.emit (events.js:203:15)
+    at endReadableNT (_stream_readable.js:1145:12)
+    at process._tickCallback (internal/process/next_tick.js:63:19)
+(node:45255) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
+(node:45255) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
+cp: docs/_dist/_headers: No such file or directory
+```
+
 - See `package-scripts.js` for details on what the builds are actually doing; especially see [markdown-magic](https://npm.im/markdown-magic) for how we're dynamically inserting information into `docs/index.md`.
 
 ## License