Merge pull request #3715 from toofar/docs/greasy-faq

Greasemonkey: add FAQ entry.

doc/faq.asciidoc

@@ -213,6 +213,37 @@ Why takes it longer to open an URL in qutebrowser than in chromium?::
     to use webengine as backend in line 17 and change it to your
     needs.
 
+How do I make qutebrowser use greasemonkey scripts?::
+    There is currently no UI elements to handle managing greasemonkey scripts.
+    All management of what scripts are installed or disabled is done in the
+    filesystem by you. qutebrowser reads all files that have an extension of
+    `.js` from the `<data>/greasemonkey/` folder and attempts to load them.
+    Where `<data>` is the qutebrowser data directory shown in the `Paths`
+    section of the page displayed by `:version`. If you want to disable a
+    script just rename it, for example, to have `.disabled` on the end, after
+    the `.js` extension. To reload scripts from that directory run the command
+    `:greasemonkey-reload`.
++
+Troubleshooting: to check that your script is being loaded when
+`:greasemonkey-reload` runs you can start qutebrowser with the arguments
+`--debug --logfilter greasemonkey,js` and check the messages on the
+program's standard output for errors parsing or loading your script.
+You may also see javascript errors if your script is expecting an environment
+that we fail to provide.
++
+Note that there are some missing features which you may run into:
+
+. Some scripts expect `GM_xmlhttpRequest` to ignore Cross Origin Resource
+  Sharing restrictions, this is currently not supported, so scripts making
+  requests to third party sites will often fail to function correctly.
+. If your backend is a QtWebEngine version 5.8, 5.9 or 5.10 then regular
+  expressions are not supported in `@include` or `@exclude` rules. If your
+  script uses them you can re-write them to use glob expressions or convert
+  them to `@match` rules.
+  See https://wiki.greasespot.net/Metadata_Block[the wiki] for more info.
+. Any greasemonkey API function to do with adding UI elements is not currently
+  supported. That means context menu extentensions and background pages.
+    
 == Troubleshooting
 
 Unable to view flash content.::