README.md: detail how to list all build options

We do not list all build options inside of the README.md, and we
definitly shouldn't do so. But in order to help people discover what can
be configured, add instructions on how to have CMake generate the list
of all knobs together with their current value.

README.md

@@ -254,6 +254,15 @@ The following CMake variables are declared:
 - `THREADSAFE`: Build libgit2 with threading support (defaults to ON)
 - `STDCALL`: Build libgit2 as `stdcall`. Turn off for `cdecl` (Windows; defaults to ON)
 
+To list all build options and their current value, you can do the
+following:
+
+	# Create and set up a build directory
+	$ mkdir build
+	$ cmake ..
+	# List all build options and their values
+	$ cmake -L
+
 Compiler and linker options
 ---------------------------