README: document requirement about python

Provide information on what is expected from the build system
regarding python.
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

README

@@ -181,6 +181,13 @@ Various tools, such as pygrub, have the following runtime dependencies:
           URL:    http://www.python.org/
           Debian: python
 
+Note that the build system expects `python` to be available. If your system
+only has `python2` or `python3` but not `python` (as in Linux From Scratch),
+you will need to create a symlink for it, or specify PYTHON= when invoking
+make, like (note the position of PYTHON= matters):
+
+    # make PYTHON=/usr/bin/python3
+
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