Add INSTALLING.md file to source distribution.

[Breakthrough]

As per title, need to include a plain-text INSTALLING.md file with installation instructions, in addition to both links to the online documentation as well as instructions on building the included documentation.

Feel free to reference/include parts of the current installation instructions on the Download page, as well as suggest changes to those as well if it makes sense.

You may also reference, use or rewrite entirely my Installing OpenCV 3.0 Guide here if it makes sense, although note that the INSTALLING file should obviously reference the most recent version of OpenCV available at the time. Note that this document is public domain and requires no attribution.

[Breakthrough]

Closing this as now everything uses the Python packages, and is distributed on PIP now. The existing documentation page referenced above is sufficient, and is included with all source distributions already.