Add additional test dependencies

These are now noted in requirements.txt

Also make dependency on 7zip and ffmpeg explicit in README

Signed-off-by: Will Beason <willbeason@gmail.com>

README.rst

@@ -12,6 +12,14 @@ submodule like::
 Wikimedia dumps are usually in a compressed format such as 7z (most common), gz, or bz2. Wikiq uses your computer's compression software to read these files. Therefore wikiq depends on
 `7za`, `gzcat`, and `zcat`. 
 
+Dependencies
+----------------
+These non-Python dependencies must be installed on your system for wikiq and its
+associated tests to work.
+
+- 7zip
+- ffmpeg
+
 
 TODO:
 _______________