Add README instructions

README.md

@@ -1,2 +1,28 @@
 # amclient
-Archivematica API client module
+
+The transfers/amclient.py script is a module and CLI that provides
+functionality for interacting with the various Archivematica APIs.
+
+Basic usage:
+    `amclient.py <subcommand> [optional arguments] <positional argument(s)>`
+
+  E.g.:
+  ```bash
+    amclient.py close-completed-transfers \
+        --am-user-name test 234deffdf89d887a7023546e6bc0031167cedf6
+  ```
+
+To see a list of all commands and how they are used, then run `amclient.py`
+without any arguments.
+
+To understand how to use an individual subcommand, simply run:
+`amclient.py <subcommand>`, the output will describe the input parameters for
+that command:
+
+  E.g.:
+  ```bash
+    usage: amclient extract-file [-h] [--ss-user-name USERNAME] [--ss-url URL]
+                                 [--directory DIR]
+                                 [--saveas-filename SAVEASFILENAME]
+                                 ss_api_key package_uuid relative_path
+  ```