[doc] install notes

README.md

 * clone source: `git clone git@github.com:asciimoo/searx.git && cd searx`
 * install dependencies: `pip install -r requirements.txt`
 * edit your [searx/settings.py](https://github.com/asciimoo/searx/blob/master/searx/settings.py) (set your `secret_key`!)
+* rename `engines.cfg_sample` to `engines.cfg`
 * run `python searx/webapp.py` to start the application
 
 ### TODO