Update readme install routine

README.md

@@ -2,6 +2,13 @@
 
 Git statistics over multiple Git repositories
 
+## Installation
+
+In order to access the script in every directory you should copy it to `/usr/local/bin`.
+```sh
+sudo cp gitistics.sh /usr/local/bin/gitistics
+```
+
 ## Usage
 
 | Option | Description |