Added crosspost warning (#1019)

* Added crosspost warning

AUTHORS.rst

@@ -21,6 +21,7 @@ Documentation Contributors
 - Watchful1 `@Watchful1 <https://github.com/Watchful1>`_
 - theonefoster `@theonefoster <https://github.com/theonefoster>`_
 - Eric Woolard `@ericwoolard <https://github.com/ericwoolard>`_
+- Marc `@PerhapsSomeone <https://github.com/PerhapsSomeone>`_
 - Add "Name <email (optional)> and github profile link" above this line.
 
 

praw/models/reddit/submission.py

@@ -272,6 +272,9 @@ def unhide(self, other_submissions=None):
     def crosspost(self, subreddit, title=None, send_replies=True):
         """Crosspost the submission to a subreddit.
 
+        .. note::
+            Be aware you have to be subscribed to the target subreddit.
+
         :param subreddit: Name of the subreddit or :class:`~.Subreddit`
             object to crosspost into.
         :param title: Title of the submission. Will use this submission's