Update docs
Update docs

Changes the following:

0. Changes key name to account.key (not account.pem)
1. Mentions that a key needs to be generated if it doesn't exist
2. Updates path to ~/.config/acme
3. Increases key size to 4096 bytes (why not)

Regarding item 1, previous docs seems to imply that `acme` will
generate a key for the user, but this doesn't seem to happen if
running the `acme reg` command just after `go get`:

```
$ acme reg mailto:me@domain.com
account key: open /home/me/.config/acme/account.key: no such file or directory
```

google/acme