Mention Memory Requirements in README

[omerXfaruq]

Hello,

When I tried to run bifrost on reference genome FASTA file with 2-4 threads and kmer size 31, the process was getting killed after some time. After analyzing the memory usage I realized 18G of memory was not enough for it to run.

It would be nice to mention total amount of memory required for some tasks in README to give some sense of it.

Currently trying kmer size 15, will let you know if it works.

Edit:
As far as I can see kmer 15 uses 12GB of memory in this setup. From that we can assume
kmer 31 -> 24GB
kmer 63 -> 48GB