Clearer genome_info description.

docs/usage.md

@@ -73,15 +73,16 @@ An [example samplesheet](../assets/samplesheet.csv) has been provided with the p
 
 ### Genomes input
 
-The second mandatory input is the genome input sheet, which needs to be specified with the option `--genomeinfo`. The file is a `.csv` file and it requires three columns: accno, genome_fna, genome_gff. it looks as follow:
+A second file input is the genome input sheet, which is specified with the option `--genomeinfo`. The file is a `.csv` file and it can contain three columns: `accno`, `genome_fna`, `genome_gff`. The first two are mandatory, while the third, `genome_gff`, is not.
 
 ```csv title="samplesheet.csv"
 accno,genome_fna,genome_gff
 GCA_002688505,./genomes/GCA_002688505.fna,./genomes/GCA_002688505.gff
 GCA_002688515,./genomes/GCA_002688515.fna,
 ```
 
-N.B.: you don't need to provide gff, Prokka can handle if you only provide fasta files but we recommend when adding gff that the file is generated by using prokka in order to don't get any conflict with other generated gffs.
+N.B. The pipeline assumes gff files have the same format as is output by Prokka.
+Any genome used by the pipeline for which a gff file is not found will be annotated with Prokka to produce a gff file.
 
 | Column       | Description                                                                                                                               |
 | ------------ | ----------------------------------------------------------------------------------------------------------------------------------------- |