Link to OpenBSD port of borgmatic.

README.md

@@ -87,6 +87,7 @@ you're using Python 3, as borgmatic does not support Python 2.
    Fedora](https://bodhi.fedoraproject.org/updates/?search=borgmatic).
  * [A borgmatic package for Arch
    Linux](https://aur.archlinux.org/packages/borgmatic/).
+ * [A borgmatic package for OpenBSD](http://ports.su/sysutils/borgmatic).
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 ## Configuration