clojure: Declare the clojure toolchain in .tool-versions

clojure/.tool-versions

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+java corretto-21
+leiningen 2.12

clojure/README.md

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+# Clojure
+
+Skeleton project for Clojure.
+
+## Prerequisites
+
+The toolchain this project needs is declared in [`.tool-versions`](./.tool-versions):
+
+```
+java corretto-21
+leiningen 2.12
+```
+
+Install those however you prefer, as long as they end up on your `PATH`. The quickest route is
+[mise](https://mise.jdx.dev/), which reads the file for you:
+
+```sh
+mise install
+```
+
+## Usage
+- `./script/setup` to fetch dependencies
+- `./script/test` to run the tests
+- `./script/start` to run the app
+
+## Structure
+- Code located in [`core.clj`](./src/pairing/core.clj)
+- Tests located in [`core_test.clj`](./test/pairing/core_test.clj)
+
+However, you're free to organise your code as you like.