Explain ast as lib goal in README.
Explain ast as lib goal in README.

DmitryOlshansky/SDC,SDC-Developers/SDC,JohanEngelen/SDC,JohanEngelen/SDC,deadalnix/SDC,bpr/SDC,SDC-Developers/SDC,DmitryOlshansky/SDC,sidmutha/SDC,schancel/SDC,deadalnix/SDC,UplinkCoder/SDC,schancel/SDC,JohanEngelen/SDC,SDC-Developers/SDC,UplinkCoder/SDC,bpr/SDC,schancel/SDC,deadalnix/SDC,UplinkCoder/SDC,bpr/SDC,DmitryOlshansky/SDC,sidmutha/SDC,sidmutha/SDC