docs(generic): add basic usage info

docs/services/generic.md

@@ -1,4 +1,31 @@
 # Generic
+The Generic service can be used for any target that is not explicitly supported by Shoutrrr, as long as it
+supports recieving the message via a POST request.
+Usually, this requires customization on the recieving end to interpret the payload that it recives, and might
+not be a viable approach.
+
+## Shortcut URL
+You can just add `generic+` as a prefix to your target URL to use it with the generic service, so
+```
+https://example.com/api/v1/postStuff
+```
+would become
+```
+generic+https://example.com/api/v1/postStuff
+```
+
+## Forwarded query variables
+All query variables that are not listed in the [Query/Param Props](#queryparam_props) section will be
+forwarded to the target endpoint.
+If you need to pass a query variable that _is_ reserved, you can prefix it with an underscore (`_`).
+
+!!! example
+    The URL `generic+https://example.com/api/v1/postStuff?contenttype=text/plain` would send a POST message
+    to `https://example.com/api/v1/postStuff` using the `Content-Type: text/plain` header.
+
+    If instead escaped, `generic+https://example.com/api/v1/postStuff?_contenttype=text/plain` would send a POST message
+    to `https://example.com/api/v1/postStuff?contenttype=text/plain` using the `Content-Type: application/json` header (as it's the default).
+
 
 ## URL Format