Manual install instruction

The install instruction in the manual still used an old `configure` setup. I copied and adjusted the instructions from `README.md`. It would be nice if a native speaker could look over it. Thanks
I do an in source build with `./acprep opt make --python -j4 -- -DPYTHON_EXECUTABLE=/usr/bin/python2`. With this I can't run any DocTests. ctests only finds RegressTest, BaselineTest, ManualTest, LedgerMathTests, LedgerUtilTests. All the tests that ctest finds pass.