Update README with usage information

README.md

@@ -1 +1,50 @@
 # ycclient
+
+A tool to fetch details of a specific module from YANG Catalog.
+
+```
+Usage of ycclient:
+  -base string
+    	Base URL for YANG catalog
+  -config string
+    	Configuration file
+  -contributors
+    	Retrieve contributing entities to the YANG catalog
+  -get
+    	Perform REST GET request to YANG catalog
+  -module string
+    	Retrieve module metadata
+  -org string
+    	Organization to retrieve
+  -revision string
+    	Module revision date
+  -vendor string
+    	Vendor to retrieve
+  -verbose
+    	Enable verbose output
+  -version
+    	Build information
+```
+
+The base url to YANG Catalog must be either passed with the `--base` argument, or specified in a toml config file like so:
+```
+[base]
+url = "https://yangcatalog.org"
+```
+
+### Examples
+Get a list of organizations that have contributed modules to YANG Catalog
+```
+ycclient --base https://yangcatalog.org --get --contributors
+```
+Fetch the details of the `Cisco-NX-OS-device@2022-08-18` YANG module
+```
+ycclient --base https://yangcatalog.org --get --module Cisco-NX-OS-device --revision 2022-08-18 --org cisco --vendor cisco
+```
+
+### Compatibility
+ycclient fetches data from the following YANG Catalog endpoints:
+- `api/contributors`
+- `api/search/modules/<name>,<revision>,<organization>`
+
+These endpoints are covered by tests on YANG Catalog's side and guaranteed to maintain compatibility.