Document Java 11 requirement, closes #511

Author: mk

README.md

@@ -42,7 +42,8 @@ ALPHA, expect breaking changes.
 
 ## 👩‍🎨 Using Clerk
 
-To use Clerk in your project, add the following dependency to your `deps.edn`:
+To use Clerk in your project, you'll need Java 11+ and [`clojure`](https://clojure.org/guides/install_clojure). Add the
+following dependency to your `deps.edn`:
 
 ```edn
 {:deps {io.github.nextjournal/clerk {:mvn/version "0.14.919"}}}

book.clj

@@ -39,6 +39,8 @@
 
 ;; ## 🚀 Getting Started
 
+;; Clerk requires Java 11 or newer and [`clojure`](https://clojure.org/guides/install_clojure) installed.
+
 ;; ### 🤹 Clerk Demo
 
 ;; When you're not yet familiar with Clerk, we recommend cloning and playing with the [nextjournal/clerk-demo](https://github.com/nextjournal/clerk-demo) repo.