Add a README with some basic instructions

"*Building GCC is not trivial, but is not difficult if you follow the instructions carefully. Many people rush into trying to build it without reading the installation docs properly and make one or more of these common mistakes:

1) do not run ./configure from gcc src dir (this is not supported) => you need to run configure from outside the gcc source directory

2) Note: if GCC links dynamically to the prerequisite libs (GMP/MPFR/MPC) then the shared libraries must be in the dynamic linker's path (LD_LIBRARY_PATH), both when building gcc and when using the installed compiler.*"