Add Gallium docs.

In Sphinx/ReST format.
diff --git a/src/gallium/docs/source/tgsi.rst b/src/gallium/docs/source/tgsi.rst
new file mode 100644
index 0000000..2474925
--- /dev/null
+++ b/src/gallium/docs/source/tgsi.rst
@@ -0,0 +1,7 @@
+TGSI
+====
+
+TGSI, Tungsten Graphics Shader Instructions, is an intermediate language
+for describing shaders. Since Gallium is inherently shaderful, shaders are
+an important part of the API. TGSI is the only intermediate representation
+used by all drivers.