Update README.md
It tooked me a while to figure out why weechat says that websocket module was not found in wee-slack plugin.
pip list sayed websocket-client was installed.
after installing pip3 and websocket-client with pip3, everything worked fine.
The current ubuntu 19.04 `python-websocket` package has always installed the python2.7 version.
@@ -30,6 +30,7 @@ Dependencies
------------
* WeeChat 1.3+ http://weechat.org/
* websocket-client https://pypi.python.org/pypi/websocket-client/
+ * Since WeeChat 2.6, Python 3 modules are required, see https://weechat.org/blog/post/2019/07/02/Python-3-by-default
* Some distributions package weechat's plugin functionalities in separate packages.
Be sure that your weechat supports python plugins. Under Debian, install `weechat-python`
