Add description of BSRAM harness
In some cases, Gowin IDE adds a number of LUTs and DFFs to the BSRAM. Here we are trying to add similar elements.
More details with pictures: https://github.com/YosysHQ/apicula/blob/master/doc/bsram-fix.md
Signed-off-by: YRabbit <rabbit@yrabbit.cyou>