README: Add pointer to Dockerfiles for easy lookup of dependencies

I spent quite some time trying to install all the dependencies to build
igt in for my distribution. After installing the packages, I noticed
that the Dockerfiles had the exact packages required for each
distribution. Add a pointer to the Dockerfiles in the README to save
folks time on setting up the environment required to build igt.

CC: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Easwar Hariharan <easwar.hariharan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

README.md

@@ -176,6 +176,8 @@ The following dependencies are requires for building audio support
 	libasound2-dev
 	libgsl-dev
 
+See Dockerfiles.* for package names in different distributions.
+
 Meson build system support
 --------------------------