Improve Safari installation doc (fix gorhill#6)

tl;dr use keyboard to press trust, not mouse

dist/README.md

@@ -11,3 +11,13 @@ For now, when installing manually, updates will not show up in Safari's extensio
 Therefore, you will have to repeat this process to update. For some users, updating manually is actually an advantage because:
 - You can update when **you** want
 - If ever a new version sucks, you can easily just re-install the previous one
+
+##### Troubleshooting
+
+If clicking *Trust* [does not work](https://github.com/el1t/uBlock-Safari/issues/6), you may [need to use keyboard controls](https://discussions.agilebits.com/discussion/57442/safari-extensions-dont-install-but-no-error-message):
+
+> 1. Launch **System Preferences** and open the **Keyboard** pane.
+> 1. Switch to the **Shortcuts** tab.
+> 1. Toggle **All Controls** to selected.
+> 1. Re-attempt to install the extension and use the `Tab` key on the keyboard to highlight **Trust**.
+> 1. Press the `Space bar` key to press the **Trust** button once selected.