chore: update readme for vulnerability database operations

Usage:
  grype db [command]

Available Commands:
  check       check to see if there is a database update available
  delete      delete the vulnerability database
  diff        diff two DBs and display the result
  import      import a vulnerability database archive
  list        list all DBs available according to the listing URL
  status      display database status
  update      download the latest vulnerability database

Flags:
  -h, --help   help for db

Global Flags:
  -c, --config string   grype configuration file
  -q, --quiet           suppress all logging output
  -v, --verbose count   increase verbosity (-v = info, -vv = debug)

Use "grype db [command] --help" for more information about a command. command

Signed-off-by: Adnan Gulegulzar <gulegulzaradnan@gmail.com>

README.md

@@ -567,6 +567,8 @@ Grype provides database-specific CLI commands for users that want to control the
 
 `grype db import` — provide grype with a database archive to explicitly use (useful for offline DB updates)
 
+`grype db providers` - provides a detailed list of database providers
+
 Find complete information on Grype's database commands by running `grype db --help`.
 
 ## Shell completion