Added a KNOWN ISSUES section to README.md

### KNOWN ISSUES
* A bug in the `DixonApp.m` wrapper causes incorrect operation when the water peak location is not entered as 0 ppm. Since this repo is supposed to be a "frozen" version of the highest scoring FattyRiot entry, the bug remains. The bug is corrected in the [fw_i3cm1i_3pluspoint_berglund_QPBO](https://github.com/welcheb/fw_i3cm1i_3pluspoint_berglund_QPBO) repo.