Add examples for searching repositories (PyGithub#874) (PyGithub#918)

doc/examples/MainClass.rst

@@ -37,4 +37,35 @@ Get organization by name
 
  >>> org = g.get_organization("PyGithub")
  >>> org.login
- u'PyGithub'
+ u'PyGithub'
+
+Search repositories by language
+-------------------------------
+
+.. code-block:: python
+
+ >>> repositories = g.search_repositories(query='language:python')
+ >>> for repo in repositories:
+ ... print(repo)
+ ...
+ Repository(full_name="vinta/awesome-python")
+ Repository(full_name="donnemartin/system-design-primer")
+ Repository(full_name="toddmotto/public-apis")
+ Repository(full_name="rg3/youtube-dl")
+ Repository(full_name="tensorflow/models")
+ Repository(full_name="django/django")
+
+Search repositories based on number of issues with good-first-issue
+-------------------------------------------------------------------
+
+.. code-block:: python
+
+ >>> repositories = g.search_repositories(query='good-first-issues:>3')
+ >>> for repo in repositories:
+ ... print(repo)
+ ...
+ Repository(full_name="vuejs/vue")
+ Repository(full_name="facebook/react")
+ Repository(full_name="facebook/react-native")
+ Repository(full_name="electron/electron")
+ Repository(full_name="Microsoft/vscode")