Update README with usage instructions

README.md

@@ -4,7 +4,42 @@ A tool for migrating Todoist tasks to Taskwarrior.
 
 ## Usage
 
-...
+Running the tool requires that your Todoist API key is available from the
+environment under the name `TODOIST_API_KEY`. The key can be found or created in
+the [Todoist Integrations Settings](https://todoist.com/prefs/integrations).
+
+The main task is `migrate` which will import all tasks. Since Todoist's internal
+ID is saved with the task, subsequent runs will detect and skip duplicates:
+
+```sh
+$ python -m todoist_taskwarrior.cli migrate --help
+Usage: cli.py migrate [OPTIONS]
+
+Options:
+  -i, --interactive
+  --no-sync
+  --help             Show this message and exit.
+```
+
+Using the `--interactive` flag will prompt the user for input for each task,
+allowing the task to be modified before import:
+
+```sh
+$ python -m todoist_taskwarrior.cli migrate --interactive
+Task 1 of 315: Work on an open source project
+
+tid: 142424242
+name: Work on an open source project
+project: Open Source
+priority:
+tags:
+entry: 2019-01-18T12:00:00+00:00
+due: 2019-01-21T17:00:00+00:00
+recur: 3 days
+```
+
+By default, `migrate` will refetch all tasks from Todoist on each run. To skip
+this step and use the cached data without refetching, use the --no-sync flag.
 
 ## Development
 
@@ -13,3 +48,4 @@ A tool for migrating Todoist tasks to Taskwarrior.
 ```sh
 $ python -m pytest tests
 ```
+