README: Mention running "./autogen.sh"
Also explain that CELT 0.5.1 isn't needed, unless someone already uses it and knows they need it. And, that if CELT 0.5.1 is installed, when running ./autogen.sh, you must explicitly disable or enable CELT 0.5.1.
Signed off by: James P. Harvey jamespharvey20@gmail.com