Update CLI usage guide, add project lookup guide

There's a new inconsistency, however. The previous URL and FQRN
arguments are no longer mandatory but their description makes it seem
as though they are.

README.md

@@ -5,7 +5,7 @@ CLI tools for interacting with the Gitea API. Use interactively to talk to your
 ## Usage
 
 ```txt
-Usage: gt-tools --url <GITEA_URL> --repo <REPO> <COMMAND>
+Usage: gt-tool [OPTIONS] <COMMAND>
 
 Commands:
   list-releases   
@@ -16,6 +16,7 @@ Commands:
 Options:
   -u, --url <GITEA_URL>    [env: GTTOOL_GITEA_URL=]
   -r, --repo <REPO>        [env: GTTOOL_FQRN=]
+  -p, --project <PROJECT>  Path to project (relative or absolute). Used to select configuration.
   -h, --help               Print help
   -V, --version            Print version
 ```
@@ -40,6 +41,12 @@ The repository name must be provided with `--repo` or `-u` on the command line,
 
 E.g.: `--repo "go-gitea/gitea"` would name the Gitea repo belonging to the go-gitea organization.
 
+### `<PROJECT>`
+
+Override the default (current-directory) project name when searching through the config files for this project's settings.
+
+See [configuration](#configuration) for details on format and file locations.
+
 ### `<COMMAND>`:
 
 One of these, defaults to `help`: