alacritty: add support for OpenBSD.

This commit expands the conditional compilation directives to support
building Alacritty for OpenBSD.  The build succeeds, and Alacritty runs
without issue once https://github.com/rust-lang/libc/pull/957 has been
merged and added to a versioned libc release.

This has been tested on the recently-released OpenBSD 6.3 on amd64 with
rustc 1.24.0 from its standard ports tree.

1 files changed, 12 insertions, 2 deletions

diff --git a/README.md b/README.md
index 2d08f8b1..caf9d721 100644
--- a/README.md
+++ b/README.md
@@ -5,8 +5,8 @@ Alacritty
 
 Alacritty is the fastest terminal emulator in existence. Using the GPU for
 rendering enables optimizations that simply aren't possible in other emulators.
-Alacritty currently supports FreeBSD, Linux, and macOS. Windows support is planned
-before the 1.0 release.
+Alacritty currently supports FreeBSD, Linux, macOS, and OpenBSD. Windows
+support is planned before the 1.0 release.
 
 <p align="center">
   <img width="600" alt="Alacritty running vim inside tmux" src="https://cloud.githubusercontent.com/assets/4285147/21585004/2ebd0288-d06c-11e6-95d3-4a2889dbbd6f.png">
@@ -158,6 +158,16 @@ missing, please open an issue.
 pkg install cmake freetype2 fontconfig xclip pkgconf
 ```
 
+#### OpenBSD
+
+Alacritty builds on OpenBSD 6.3 almost out-of-the-box if Rust and
+[Xenocara](https://xenocara.org) are installed.  If something is still found to
+be missing, please open an issue.
+
+```sh
+pkg_add rust
+```
+
 #### Solus
 
 On [Solus](https://solus-project.com/), you need a few extra libraries to build