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
`ctest` needs to be run from ledger's build directory, the same directory where the `ledger` binary and `ledger.so` can be found, for me this is `build`, so I need to run `(cd build; ctest -R DocTests)`, alternatively you could also try `./acprep check`.