add doc about knowit installation

README.rst

@@ -93,6 +93,8 @@ clone your fork locally and install a development version::
     $ cd subliminal
     $ pip install --user -e '.[dev,test,docs]'
 
+To extract information about the video files, `subliminal` uses `knowit <https://github.com/ratoaq2/knowit`.
+For better results, make sure one of its provider is installed, for instance `MediaInfo <https://mediaarea.net/en/MediaInfo>`.
 
 Nautilus/Nemo integration
 -------------------------

docs/index.rst

@@ -15,6 +15,7 @@ Documentation
     :maxdepth: 2
 
     user/usage
+    user/installation
     user/how_it_works
     user/cli
     user/provider_guide

docs/user/installation.rst

@@ -0,0 +1,30 @@
+Installation
+============
+
+From Pypi
+---------
+
+For a better isolation with your system you should use a dedicated virtualenv.
+The preferred installation method is to use `pipx <https://github.com/pypa/pipx>`_ that does that for you::
+
+    $ pipx install subliminal
+
+Subliminal can be also be installed as a regular python module by running::
+
+    $ pip install --user subliminal
+
+From source
+-----------
+
+If you want to modify the code, `fork <https://github.com/Diaoul/subliminal/fork>`_ this repo,
+clone your fork locally and install a development version::
+
+    $ git clone https://github.com/<my-username>/subliminal
+    $ cd subliminal
+    $ pip install --user -e '.[dev,test,docs]'
+
+External dependencies
+---------------------
+
+To extract information about the video files, `subliminal` uses `knowit <https://github.com/ratoaq2/knowit`.
+For better results, make sure one of its provider is installed, for instance `MediaInfo <https://mediaarea.net/en/MediaInfo>`.