Update README.md with build instructions

[skip ci]

Signed-off-by: Ash <73482956+ascopes@users.noreply.github.com>

Author: ascopes

README.md

@@ -345,7 +345,46 @@ holding the flags you wish to use.
    has a high churn of objects. On Java 11, the ZGC is an experimental feature, which needs
    to be enabled with `-XX:+UnlockExperimentalOptions -XX:+UseZGC`. On Java 17, you just
    need to pass `-XX:+UseZGC` alone.
-  
+   
+## Building this project
+
+This project uses Java 11, and bootstraps Maven using the Maven Wrapper tooling. This means that
+all you need to build this is any JDK from Java 11
+onwards: everything else will be set up automatically for you.
+
+### Non-Windows
+
+On Linux, BSD, and MacOS, I tend to use [sdkman!](https://sdkman.io/) to install the JDK I want to use:
+
+```shell
+# Install sdkman
+$ curl -s "https://get.sdkman.io" | bash
+$ source "$HOME/.sdkman/bin/sdkman-init.sh"
+
+# See what versions of Java are available
+$ sdk list java
+
+# Install a JDK of my choice, like Amazon Corretto
+$ sdk install java x.y.z-amzn
+```
+
+Once that is configured, you can just run the Maven Wrapper shell script to download, install,
+and run the correct version of Maven:
+
+```shell
+$ ./mvnw clean package
+```
+
+### Windows
+
+On Windows, you can install the JDK of your choice directly from their website, or use Chocolatey to
+install it for you. Once installed, you can just run the Maven Wrapper batch script to download,
+install, and run the correct version of Maven:
+
+```cmd
+.\mvnw.cmd clean package
+```
+
 ## ECJ support
 
 While this module initially supported ECJ, there were a number of problems relating to bugs