As nheko does not support yet sending encrypted attachments (see issue #348) explicitly indicate so in the README.

fixes #345

[ci skip]

fixes #264

fixes #226

[ci skip]

Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

The first distribution to have nheko in it's official repositories! 

    Right now nheko is only in the `testing` repository, but this will
    be moved to `community` after a while (after which it's available
    for installation out of the box).