How-to-guide.md

+## Define a root to build the taxonomy tree
+
+By default, MKBDR load the NCBI taxonomy tree using "vertebrae" as a root taxon ( Taxon ID = 7742).
+It is possible to define a different root taxon to load the taxonomy tree using the option `--root_ncbi_taxon_id`.
+
+For instance if you are working on plants, you can use the node "Viriplantae" (Taxon ID = 33090) as a root to load your ncbi taxonomy tree.
+
+```
+mkbdr validate --fasta test_its2.fasta -o "viriplantae" -r 33090
+```
+
+
 ## Fix issue: `mkbdr curegen` sqlite3.OperationalError
 
 The first time you run `mkbdr curegen` you may have this error: