README update.
DRILL-1411 Fragment memory planning
DRILL-1391 : Fix compilation error in run-time generated code for several math functions over decimal types.
DRILL-1383: Support interpreted execution for Drill expression tree.

Fix OOM while running InterpreterBuilder
Patch for DRILL-705

Currently only supports partitioning/ordering, not yet preceding or
after offsets
DRILL-1386: Push directory filters into group scan for filesystem queries.