Document the exact range of supported Racket versions in README

related #167

Author: dannypsnl

README.md

@@ -4,7 +4,7 @@
 
 ## Installation and usage
 
-A Racket runtime is a prerequisite, so before using `racket-langserver`, ensure that a Racket runtime is installed. You can install an from the [official download page](https://download.racket-lang.org) or install one from your package manager.
+A Racket runtime is a prerequisite, so before using `racket-langserver`, ensure that a Racket runtime is installed. You can install an from the [official download page](https://download.racket-lang.org) or install one from your package manager. Currently `racket-langserver` is compatible with Racket version 7.6 to 9.0.
 
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