message: Add Slackware section to README.md
diff: @@ -100,6 +100,13 @@ still found to be missing, please open an issue.
  zypper install cmake freetype-devel fontconfig-devel xclip
  ```
  +##### Slackware
  +
  +Compiles out of the box for 14.2
  +For copy & paste support (middle mouse button) you need to install xclip
  +https://slackbuilds.org/repository/14.2/misc/xclip/?search=xclip
  +
  +
  ##### Void Linux
  On [Void Linux](https://voidlinux.eu), install following packages before compiling Alacritty:
  