Add suggestion for lua headers inclusion issues

On some systems "latests" lua header files are located in /usr/include,
while the header files for older lua versions can be found in subdirectories
such as /usr/include/lua5.1 or similar. In those situations, despite compiling
for an older lua version, CMake will include the headers from /usr/include
resulting in a broken build only detectable at runtime.

Since lua headers locations are not standardized, patching .cmake files
is pointless, while adding a suggestion in the docs seems to be the right
thing to do.

Signed-off-by: Davide "FunkyAss" Del Zompo <davide.delzompo@gmail.com>
Merge pull request #8 from FunkyAss/gh-pages-luadir

Add suggestion for lua headers inclusion issues