Updates the documentation

docs/usage.md

@@ -21,6 +21,7 @@ This section describes how to use *pyRona*. All options as of version 0.3.3 are
             - `-fc`: File containing future environmental variables, formatted as per the upstream program input (*Baypass* or *LFMM*)
             - `-out`: File where the plot will be saved. This option is extension aware, and entering the extension as "PDF", "SVG" or "PNG", will make *pyRona* save the plot in the respective format
             - `-map`: File where a RONA map will be saved. This options is also extension aware. In order to draw the map, `cartopy` must be installed on your system (see [Installation](install.md)) `-map` assumes that the first variable in the environmental file is "Latitude" and the second one is "Longitude"
+            - `-covar_names`: File containing the name of each environmental variable. Use it if you want your plots to be named accoring to the used variables, instead of numbered. Each line of this file should contain a variable name in the same order as in the covariates file
         - *Baypass* specific options:
             - `-pop`: File with names of populations (one population per line)
             - `-beta`: *BayPass* output "summary_betai"