Makefile: add description on PERL/PYTHON_PATH

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -241,11 +241,16 @@ all::
 # apostrophes to be ASCII so that cut&pasting examples to the shell
 # will work.
 #
+# Define PERL_PATH to the path of your Perl binary (usually /usr/bin/perl).
+#
 # Define NO_PERL_MAKEMAKER if you cannot use Makefiles generated by perl's
 # MakeMaker (e.g. using ActiveState under Cygwin).
 #
 # Define NO_PERL if you do not want Perl scripts or libraries at all.
 #
+# Define PYTHON_PATH to the path of your Python binary (often /usr/bin/python
+# but /usr/bin/python2.7 on some platforms).
+#
 # Define NO_PYTHON if you do not want Python scripts or libraries at all.
 #
 # Define NO_TCLTK if you do not want Tcl/Tk GUI.