[ci skip] Update docs (#66)

docs/CONTRIBUTING.md

@@ -3,4 +3,72 @@ Contributing to Leaves
 
 **English** | [中文](https://github.com/LeavesMC/Leaves/blob/master/docs/CONTRIBUTING_cn.md)
 
-TODO
+We're glad that you want to contribute to our project!  
+In general, our review of pull requests is very lenient.  
+And if you can follow the rules below, we can complete the review faster.
+
+## Please fork using your personal account
+
+We regularly merge existing PRs.  
+If there're some small problems, we'll help you solve them by editing your PR.
+
+But, if your PR is from a organization, we can NOT edit your PR, so we must merge your PR manually.
+
+So, don't use orgnization accounts for fork!
+
+See also [This issue](https://github.com/isaacs/github/issues/1681), and then you'll know why we can't edit PRs from organizations.
+
+## Development Environment
+
+Before coding, you need these softwares / tools as Dev Environment.
+
+- `git`
+- `JDK 17 or higher`
+  - We used Gradle's toolchains, so you can build Leaves using JRE 8. (When Gradle can't find JDK 17, it will download it automatically.)
+
+If you're using Windows Operating System, you can use `WSL` to speed up building.
+
+## Understanding "Patches"
+
+Leaves uses as the same patching system as Paper,  
+and has been divided into two directories for the purpose of modifying different parts of it:
+
+- `leaves-api` - Modifications to `Paper-API` / `Spigot-API` / `Bukkit`.
+- `leaves-server` - Modifications to `Paper` / `Spigot` / `CraftBukkit`.
+
+The patching system is based on git, and you can learn about it at here: <https://git-scm.com/docs/gittutorial>
+
+If you have forked the main repository, then you should follow the steps below:
+
+1. Clone your repository to local
+2. Run Gradle's `applyPatches` task in your IDE or terminal (You can run `./gradlew applyPatches` directly in terminal.)
+3. Enter `leaves-api` or `leaves-server` directory to carry out modifications.
+
+BTW, `leaves-api` and `leaves-server` are not normal git repositories.
+
+- Before applying patches, the base will point to unmodified source code.
+- Every commit after the base is a patch.
+- Only commits after the last commit of Paper will be considered Leaves' patches.
+
+## Adding new patches
+
+It's very easy to to add patches by following the steps below:
+
+1. Modify the code of `leaves-api` and / or `leaves-server`
+2. Add these changes to the local git repository (For example, `git add .`)
+3. Commit these changes using `git commit -m <Commit Message>`
+4. Run Gradle's task `rebuildPatches` to convert your commits to a new patch
+5. Push your patches to your repository
+
+After pushing, you can open a PR to submit your patches.
+
+## Modifying patches
+
+You can modify a existing patch by following the steps below:
+
+1. Modify code at HEAD
+2. Run `git commit -a --fixup <hash>` in your terminal to make a fix-up commit
+    - If you want to edit the commit message, replace `--fixup` with `--squash`.
+3. Run `git rebase -i --autosquash base` to rebase automatically, then just type `:q` to close the confirm page
+4. Run Gradle's task `rebuildPatches` to modify existing patches
+5. Push and PR again