building.md: Add details to swig version requirements for nodejs support. resolves #1

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

docs/building.md

@@ -4,6 +4,11 @@ Building libmraa                         {#building}
 libmraa uses cmake in order to make compilation relatively painless. Cmake runs
 build out of tree so the recommended way is to clone from git and make a build/ directory.
 
+You'll need swig version 3.0.1+, currently I'm using
+8d226e39dc7a958013ff9ffd15e231ef206ba265 from the swig master branch. This is
+only required for javascript support and can be disabled with a cmake define
+(see below).
+
 ~~~~~~~~~~~~~{.sh}
 mkdir build
 cd build
@@ -25,3 +30,5 @@ Building debug build:
 Using clang instead of gcc:
  -DCMAKE_C_COMPILER=/usr/bin/clang -DCMAKE_CXX_COMPILER=/usr/bin/clang
 
+Building with an older version of swig (swig 2.0+) requires the disabling of javascript:
+ -DBUILDSWIGNODE=OFF