Fix README.md install instructions (#2136)

To install the dependencies written in a text file the `-r` flag must be passed to pip

README.md

@@ -21,7 +21,7 @@ You can clone this repo and simply execute:
 ```bash
 $ git clone git://github.com/ninja-ide/ninja-ide.git
 $ cd ninja-ide
-$ pip install requirements.txt
+$ pip install -r requirements.txt
 $ python ninja-ide.py
 ```