add a Contributors.md

closes #9

docs/Contributing.md

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+# Contributing
+
+So, you've decided to contribute to JvmDowngrader? Great! 
+This document will explain how the project is structured, and some common issues that may arise.
+
+## Project Setup
+
+1. Clone the repository
+2. build the project with `./gradlew build`
+
+## Outputs
+
+JvmDowngrader has multiple subProjects,
+- The root project exports a `-all` jar, which is the CLI artifact.
+- `java-api` is the api stubs for newer java versions.
+- `gradle-plugin` is the gradle plugin.
+- `downgradetest` is a project that builds a jar for Integration testing. this is used by the tests to run all it's `main` methods.
+    - after running the tests, the downgraded jar from the tests will also be in that folder in case it needs to be inspected.
+
+It is recommended that you use `publishToMavenLocal` if you need to test a fix for a problem discovered in another project.
+
+## Stub Guidelines
+
+For licensing reasons, please do not directly copy code from the JDK.
+
+# Troubleshooting
+
+### Unable to download toolchain matching the requirements ({languageVersion=7
+
+JvmDowngrader requires java 7 to build `src/main` and `src/shared` as they target java 7, and access `sun.misc.Unsafe` 
+which only has ct.sym entries for java 9+ and `java-8-unsupported-shim` is not compatible with java 7.
+
+The recommended solution is to manually install java 7. If this is not possible for some reason, in [`gradle.properties`](../gradle.properties)
+you can change the `mainVersion` from `7` to `8`, and the `sharedVersion` from `7` to `8`. Just be aware for contributing of which
+APIs are available in java 7, most notably the lack of lambdas and streams.
+
+### Provisioned Toolchain could not be probed
+
+This usually indicates that gradle tried to use a broken installation of java.
+delete the provisioned toolchain from the `GRADLE_HOME/jdks` directory.
+
+If the issue persists, The easiest fix is to manually install the java version stated in the error message.
+
+You can also try modifying [the build gradle](../java-api/build.gradle.kts) to use a different java vendor, as shown in the
+[Gradle documentation](https://docs.gradle.org/current/userguide/toolchains.html#sec:configuring_jvm_toolchain)
+just search for each instance of `javaToolchains` in that file.
+
+### No matching variant of com.gradleup.nmcp:nmcp:0.0.7 was found.
+
+Gradle requires using *at least* java 11 to run. you will have to change the default java binary on your system, or 
+change the version that gradle uses in some other way.
+
+#### IDEA
+
+Changing this in Intellij Idea is fairly simple. In the gradle tab on the right, click on the ⚙ (settings) icon.
+Then, in the "Gradle JVM" dropdown, select the java 11 version you have installed, or any newer java version, such as java 17 or 21.