Add documentation for user database construction

[Former user]

I was not able to find documentation to build a user-defined database instead of using the GS database. I want to use the faser algorithm within the mifaser framework, but would like added flexibility to build my own database (particularly UniRef90).

[Max Miller]

We are working on providing alternative databases; for that purpose we will add a script to generate the required database for alternative selections of reference sequences - this should be done within the coming week.

[Max Miller]

We just released version 1.49 which allows to create a user defined database (using argument -D/--createdb). Creating a new reference database requires a multi-FASTA file containing annotations and e.c-numbers in the header using the format:

>uniprot_id|annotation|e.c.-number|additional_details

than run mi-faser with:

$ python mifaser.py -D my_database path/to/sequences.fasta