Add Contributing.md for developers and advanced users

Contributing.md

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+Contributing to PaperSpigot
+==========================
+PaperSpigot has a very lenient policy towards PRs, but would prefer that you try and adhere to the following guidelines.
+
+## Understaing Patches
+Patches to PaperSpigot are very simple, but center around the directories 'PaperSpigot-API' and 'PaperSpigot-Server'
+
+Assuming you already have forked the repository:
+
+1. Pull the latest changes from the main repository
+2. Type `./applyPatches.sh` in git bash to apply the changes from upstream
+3. cd into `PaperSpigot-Server` for server changes, and `PaperSpigot-API` for api changes
+
+These directories aren't git repositories in the traditional sense:
+
+- Every single commit in PaperSpigot-Server/API is a patch. 
+- 'origin/master' points to a directory similar to PaperSpigot-Server/API but for PaperSpigot
+- Typing `git status` should show that we are 10 or 11 commits ahead of master, meaning we have 10 or 11 patches that PaperSpigot and Spigot don't
+  - If it says something like `212 commits ahead, 207 commits behind`, then type `git fetch` to update spigot/paperspigot
+
+## Adding Patches
+Adding patches to PaperSpigot is very simple:
+
+1) Modify `PaperSpigot-Server` and/or `PaperSpigot-API` with the appropriate changes
+2) Type `git add .` to add your changes
+3) Run `git commit` with the desired patch message
+4) Run `./rebuildPatches.sh` in the main directory to convert your commit into a new patch
+5) PR your patches back to this repository
+
+Your commit will be converted into a patch that you can then PR into PaperSpigot
+
+## Modifing Patches
+Modifiying previous patches is a bit more complex:
+
+1. Make sure `git status` is correct
+  - If it says something like `212 commits ahead, 207 commits behind`, then type `git fetch` to update spigot
+2. If you have changes you are working on type `git stash` to store them for later
+  - Later you can type `git stash pop` to get them back
+3. Type `git rebase -i`
+  - It should show something like [this](https://gist.github.com/Zbob750/e6bb220d3b734933c320)
+4. Replace `pick` with `edit` for the commit/patch you want to modify, and "save" the changes
+  - Only do this for one commit until you get more advanced and understand what `git rebase -i` does
+5. Make the changes you want to make to the patch
+6. Type `git add .` to add your changes
+7. Type `git commit --amend` to commit
+  - **MAKE SURE TO ADD `--amend`** or else a new patch will be created
+  - You can also modify the commit message here
+8. Type `git rebase --continue` to finish rebasing
+9. Type `./rebuildPatches.sh` in the main directory
+  - This will modify the appropriate patches based on your commits
+10. PR your modifications to github
+
+## PR Policy
+We'll accept changes that make sense. You should be able to justify their existence, along with any maintenance costs that come with them. Remember, these changes will affect everyone who runs Paper, not just you and your server.
+While we will fix minor formatting issues, you should stick to the guide below when making and submitting changes.
+
+## Formatting
+All modifications to non-PaperSpigot files should be marked
+- Multi line changes start with `// PaperSpigot start` and end with `// PaperSpigot end`
+- You can put a messages with a change if it isn't obvious, like this: `// PaperSpigot start - reason
+  - Should generaly be about the reason the change was made, what it was before, or what the change is
+  - Multi-line messages should start with `// PaperSpigot start` and use `/* Multi line message here */` for the message itself
+- Single line changes should have `// PaperSpigot` or `// PaperSpigot - reason`
+- For example:
+````java
+entity.getWorld().dontbeStupid(); // PaperSpigot - was beStupid() which is bad
+entity.getFriends().forEach(Entity::explode());
+entity.a();
+entity.b();
+// PaperSpigot start - use plugin-set spawn
+// entity.getWorld().explode(entity.getWorld().getSpawn());
+Location spawnLocation = ((CraftWorld)entity.getWorld()).getSpawnLocation();
+entity.getWorld().explode(new BlockPosition(spawnLocation.getX(), spawnLocation.getY(), spawnLocation.getZ()));
+// PaperSpigot end
+````
+- We generally follow usual java style, or what is programed into most IDEs and formatters by default
+  - This is also known as oracle style
+  - It is fine to go over 80 lines as long as it doesn't hurt readability
+  - There are exceptions, especially in Spigot-related files
+  - When in doubt, use the same style as the surrounding code