softpipe: Document new llvm flag

Signed-off-by: Jakob Bornecrantz <wallbraker@gmail.com>

docs/envvars.html

@@ -140,6 +140,8 @@ Mesa EGL supports different sets of environment variables.  See the
 <li>SOFTPIPE_DUMP_GS - if set, the softpipe driver will print geometry shaders
     to stderr
 <li>SOFTPIPE_NO_RAST - if set, rasterization is no-op'd.  For profiling purposes.
+<li>SOFTPIPE_USE_LLVM - if set, the softpipe driver will try to use LLVM JIT for
+    vertex shading procesing.
 </ul>