Required versions of dependencies

[Eli Zaretskii]

It seems we don't say anywhere what minimal versions of major external
packages are required to successfully build GDB.  I mean libexpat and
Python.  For Python, I can see in configure.ac that version 2.4 is the
oldest we test for, but I think someone who is going to build GDB
should be able to read these requirements in README, they should not
need to look around in configury stuff for that.

Can we please add this information to README?

TIA