gallium/docs - add OpenSWR documentation

Acked-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/docs/source/drivers/openswr/usage.rst

@@ -0,0 +1,38 @@
+Usage
+=====
+
+Requirements
+^^^^^^^^^^^^
+
+* An x86 processor with AVX or AVX2
+* LLVM version 3.6 or later
+
+Building
+^^^^^^^^
+
+To build with GNU automake, select building the swr driver at
+configure time, for example: ::
+
+  configure --with-gallium-drivers=swrast,swr
+
+Using
+^^^^^
+
+On Linux, building will create a drop-in alternative for libGL.so into::
+
+  lib/gallium/libGL.so
+
+or::
+
+  build/foo/gallium/targets/libgl-xlib/libGL.so
+
+To use it set the LD_LIBRARY_PATH environment variable accordingly.
+
+**IMPORTANT:** Mesa will default to using llvmpipe or softpipe as the default software renderer.  To select the OpenSWR driver, set the GALLIUM_DRIVER environment variable appropriately: ::
+
+  GALLIUM_DRIVER=swr
+
+To verify OpenSWR is being used, check to see if a message like the following is printed when the application is started: ::
+
+  SWR detected AVX2
+