building.md: add more building information

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

docs/building.md

@@ -4,10 +4,16 @@ 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).
+You'll need swig version 3.0.2 and we recommend 3.0.3 to build the javascript
+modules. If you're version of SWIG is older than this then please see below for
+disabling SWIGNODE otherwise you will get a weird build failure.
+
+The basic build steps are as follow, we'll assume you're building from git,
+note that if you compile with git installed your version of mraa will be tagged
+-dirty. This simply means git wasn't installed or that you where building form
+a tarball. You can modify build/src/version.c before running make if this is
+incorrect. The instructions listed here all assume that build/ is an empty dir
+that lives inside the cloned repository of mraa.
 
 ~~~~~~~~~~~~~{.sh}
 mkdir build
@@ -16,20 +22,23 @@ cmake ..
 make
 ~~~~~~~~~~~~~
 
-Our cmake configure has a number of options, cmake-gui or ccmake can show you
-all the options. The interesting ones are detailed below:
+Our cmake configure has a number of options, cmake-gui or ccmake (cmake -i is
+no longer with us :() can show you all the options. A few of the more common
+ones are listed below - note that when the option starts with CMAKE_ it's an
+option that is made available by cmake and will be similar in all cmake
+projects. You need to add them after `cmake` but before `..`.
 
-Few recommended options:
+A few recommended options:
 Changing install path from /usr/local to /usr
  -DCMAKE_INSTALL_PREFIX:PATH=/usr
 
-Building debug build:
+Building debug build - adds -g and disables optimisations - this will force a full rebuild:
  -DCMAKE_BUILD_TYPE=DEBUG
 
 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:
+Building with an older version of swig (swig < 3.0.2) requires the disabling of javascript:
  -DBUILDSWIGNODE=OFF
 
 Disabling python module building