Add unzip as a requirement in the contributing documentation. (#3273)

unzip is missing from documentation and is required to run 
"gclient sync"  successfully. Unzip does not come installed by default
on every Linux installations.
Signed-off-by: NPhilippe Ombredanne <pombredanne@nexb.com>

CONTRIBUTING.md

@@ -12,7 +12,7 @@ Things you will need
  * An ssh client (used to authenticate with GitHub).
  * Chromium's [depot_tools](http://www.chromium.org/developers/how-tos/install-depot-tools) (make sure it's in your path). We use the `gclient` tool from depot_tools.
  * Python (used by many of our tools, including 'gclient').
- * curl (used by `gclient sync`).
+ * curl and unzip (used by `gclient sync`).
 
 You do not need [Dart](https://www.dartlang.org/downloads/linux.html) installed, as a Dart tool chain is automatically downloaded as part of the "getting the code" step. Similarly for the Android SDK, it's downloaded by the `gclient sync` step below.