add language support for PyCharm - update docs

Author: waszil

docs/language.rst

@@ -651,3 +651,9 @@ Vim
 A syntax plugin for `vim <https://www.vim.org/>`_ is available from
 `bitproto's github repository <https://github.com/hit9/bitproto/tree/master/editors/vim>`_.
 This plugin only supports syntax highlighting of bitproto language.
+
+PyCharm
+"""""""
+
+Syntax highlighting settings for PyCharm is available from
+`bitproto's github repository <https://github.com/hit9/bitproto/tree/master/editors/pycharm>`_.