Update README.md (#1)

Add build instructions, update c library url.
SWIG build has '-threads' argument for recent GIL patch.

README.md

@@ -1,6 +1,6 @@
 # RPi WS281x Python
 
-This is the official Python distribution of the ws281x library: http://github.com/richardghirst/rpi_ws281x
+This is the official Python distribution of the ws281x library: https://github.com/jgarff/rpi_ws281x
 
 # Installing
 
@@ -11,3 +11,22 @@ Most users should simply run:
 ```
 sudo pip install rpi_ws281x
 ```
+
+## Building
+
+Clone with submodules, and enter library directory:
+```
+git clone --recurse-submodules https://github.com/rpi-ws281x/rpi-ws281x-python.git
+cd rpi-ws281x-python/library
+```
+To rebuild SWIG files if needed ("black" for code re-formatting only):
+```
+swig -python -threads rpi_ws281x.i
+black rpi_ws281x.py
+```
+
+Build and Install:
+```
+python3 setup.py build
+sudo python3 setup.py install
+```