Mention OpenSSL dependency

README.md

@@ -45,6 +45,12 @@ pip install pusher
 
 Users on Python 2.x and older versions of pip may get a warning, due to pip compiling the optional `pusher.aiohttp` module, which uses Python 3 syntax. However, as `pusher.aiohttp` is not used by default, this does not affect the library's functionality. See [our Github issue](https://github.com/pusher/pusher-http-python/issues/52), as well as [this issue from Gunicorn](https://github.com/benoitc/gunicorn/issues/788) for more details.
 
+On Linux, you must ensure that OpenSSL is installed, e.g. on Debian/Ubuntu:
+
+```sh
+$ sudo apt-get install build-essential libssl-dev libffi-dev
+```
+
 ## Getting started
 
 The minimum configuration required to use the Pusher object are the three