Add basic Makefile build info

Building-mruby.md

@@ -4,7 +4,16 @@ multi-platform or cross compiling build support.
 
 ## Building with mruby's basic Makefile
 
-_in process_
+**Prerequisites:**
+* Bison and a build toolchain on `PATH`
+
+Building with the Unix-style basic Makefiles is as you would expect. By default, a simple `make`
+builds everything and installs executables into the project's `bin/` subdir, and libraries into
+the project's `lib/` subdir. Valid build targets are listed by running `make help`. Interesting
+build targets include:
+
+1. `make clean all`
+2. `make test`
 
 ## Building with CMake