add ccache

COMPILING.md

@@ -5,3 +5,4 @@ Horizon OC Compilation Instructions
 3. Clone the Horizon OC develop branch (``git clone https://github.com/Horizon-OC/Horizon-OC.git --recurse-submodules``)
 4. Run ``./build.sh`` in the root directory
  - If you want to compile with extensions, append ``--ext``
+ - Installing ``ccache`` is optional; if it is on your PATH it is used automatically to speed up rebuilds