Decrypting doesn't write the new file

Your docs say this:

`keybase decrypt foo.txt.asc # foo.txt.asc -> foo.txt`

which makes it sound like running that command will decrypt foo.txt.asc and then write foo.txt as a file. But it doesn't actually write the file—I had to add `> foo.txt` to the command.

I'd suggest either updating the documentation or changing the behavior as they currently don't match.

Start with the `keybase decrypt` command and compare its observed behavior with the documentation example for `foo.txt.asc`. Determine whether the command is intended to create `foo.txt` or require shell redirection, then verify that the documentation and behavior agree.

Written by the indexing model from the issue text.