Update README.md

trim carriage-return at the end of signature comments

This commit fixes a bug when parsing minisign signatures.
Before, any carriage return ('\r') at the end of a
trusted / untrusted comment was not removed. This lead to
signature verification failures. In particular, the verification
of the comment signature fails when e.g. the OS uses `\r\n` for
newlines instead of `\n`.

Byte representation of a trusted comment - with and without the
trailing carriage return.
```
[116 105 109 101 115 116 97 109 112 58 49 54 49 55 55 50 49 48 50 51 9 102 105 108 101 58 114 111 98 116 101 115 116 46 112 115 49 13]
[116 105 109 101 115 116 97 109 112 58 49 54 49 55 55 50 49 48 50 51 9 102 105 108 101 58 114 111 98 116 101 115 116 46 112 115 49]
```

Fixes aead#7

cli: create directories if they don't exist.

This commit fixes an issue when generating
keys are signing a files with a custom signature
file.

Before, key generation resp. signing failed
if the destination file would have been
written to a directory that doesn't exist.

Now, minisign will create any non-existing
directory, if necessary.

cli: fix bug when generating key pairs.

This commit fixes a file creation bug in
the key generation code. If the force
flag has not been specified the OS file
flags would only contain `O_EXCL` such that
creation attempts will fail.

Now, the code *additionally* sets the `O_EXCL`
flag but preserves all other flags - i.e. `O_CREATE`.