Minor README.md issues

[eero-t]

• Prerequisites: openbox? (*)
• "Video Processing Features" section has few items marked with *, but there's no explanation on what * indicates
• "Please, refer to i915 kernel mode driver documentation to learn how to enable it" => kernel command line option "i915.enable_guc=2" for HuC loading can be set e.g. by doing echo "options i915 enable_guc=2" > /etc/modprobe.d/i915.conf (as root)
• TGL bug link is nowadays: https://gitlab.freedesktop.org/drm/intel/issues/643

(*) I use Ubuntu 19.10 container to build git versions of libdrm, libva, gmmlib & media-driver. Deps I use are:
autoconf automake libtool build-essential pkg-config cmake meson ninja-build valgrind git ca-certificates libpciaccess-dev

• libdrm is needed by libva
• ninja is to have faster make
• ca-certificates is needed for Git HTTPS access
• valgrind (headers) are there just to avoid lots of error being reported when valgrinding the resulting libraries

[XinfengZhang]

• libva already is dependency, and libdrm is dependency of libva, valgrind headers should be added to avoid noise, but need support in code and should related with Heap corruption with format conversion #715
• already was resolved https://github.com/intel/media-driver#video-processing-features
• fixed by clarify the huc enabling in readme #1007.
• it is closed.