Update README with the supported platforms

These platforms are Linux and Mac. Windows is
still not supported.

Signed-off-by: Santiago Dueñas <sduenas@bitergia.com>

Author: sduenas

README.md

@@ -60,6 +60,7 @@ Run 'perceval <backend> --help' to get information about a specific backend.
 
 ## Requirements
 
+ * Linux/Mac
  * Python >= 3.7
  * Poetry >= 1.2
  * git