add caution for mixing taxonomy

Author: jaebeom-kim

README.md

@@ -342,6 +342,8 @@ metabuli updateDB <NEW DBDIR> <FASTA_LIST> <accession2taxid> <OLD DBDIR> [option
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 ### Add sequences of new taxa
+> NOTE: Mixing taxonomies within the same domain is not recommended. For example, adding prokaryotes to a GTDB database using NCBI taxonomy will cause issues, but adding eukaryotes or viruses with NCBI taxonomy is fine since GTDB does not cover them.
+ 
 You can use `--new-taxa` option to add new taxa to the taxonomy tree.
 
 #### 1. Check if you need to add new taxa