Update README

Add OpenBSD to supported list for meminfo collector[0].

[0]: prometheus#724

README.md

@@ -41,7 +41,7 @@ infiniband | Exposes network statistics specific to InfiniBand and Intel OmniPat
 ipvs | Exposes IPVS status from `/proc/net/ip_vs` and stats from `/proc/net/ip_vs_stats`. | Linux
 loadavg | Exposes load average. | Darwin, Dragonfly, FreeBSD, Linux, NetBSD, OpenBSD, Solaris
 mdadm | Exposes statistics about devices in `/proc/mdstat` (does nothing if no `/proc/mdstat` present). | Linux
-meminfo | Exposes memory statistics. | Darwin, Dragonfly, FreeBSD, Linux
+meminfo | Exposes memory statistics. | Darwin, Dragonfly, FreeBSD, Linux, OpenBSD
 netdev | Exposes network interface statistics such as bytes transferred. | Darwin, Dragonfly, FreeBSD, Linux, OpenBSD
 netstat | Exposes network statistics from `/proc/net/netstat`. This is the same information as `netstat -s`. | Linux
 sockstat | Exposes various statistics from `/proc/net/sockstat`. | Linux