building.md: add python3 & buildarch info

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

docs/building.md

@@ -62,6 +62,14 @@ Building doc, this will require [SPHINX](http://sphinx-doc.org) &
 [Doxygen](http://doxygen.org):
  -DBUILDDOC=ON
 
+Building with python3 (careful you need to clear cmake cache between python
+version switches!)
+ -DBUILDPYTHON3=ON
+
+Override build architecture (this is useful because on x86 arm code is not
+compiled so use this flag to force the target arch)
+ -DBUILDARCH=arm
+
 ## Dependencies continued
 
 You'll need at least SWIG version 3.0.2 and we recommend 3.0.5 to build the