add list, show commands

The `list` command can be omitted and is the default. The `show` command
retains past output.

```console
$ anissue -h
Usage:
  anissue
  anissue list
  anissue show
```

The implementation uses docopts, the shell variant of docopt [1].
Because of that, wrapping with `--argv0 ''` is necessary when packaging.

[1] http://docopt.org/

1 files changed, 9 insertions, 1 deletions

diff --git a/README.md b/README.md
index b5c8079..30e5135 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,14 @@ nix-env -f. -i
 
 ## usage
 
+### list
+
+```console
+anissue list
+```
+
+### show
+
 ```console
-anissue .
+anissue show
 ```