Add contributing guidelines and issue template

.github/CONTRIBUTING.md

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+Contributing to ProtocolLib
+===========================
+This page will detail specific things that must be done if you intend to contribute to this project.
+
+## Updating ProtocolLib to a New Protocol Version
+#### Before we get started:
+1. **When do we need to update the protocol version?**
+    _A Minecraft protocol version is a number for each specification of the Minecraft protocol.
+    This means it is incremented when there is a change to the [Protocol Specification](http://wiki.vg/Protocol),
+    but not necessarily for each Minecraft version update. If the latest Minecraft version contains a protocol change,
+    that means ProtocolLib needs to be updated._
+2. **When do we need to update other parts of ProtocolLib?**
+    _When the package version guard changes. The package version guard is the version string inserted
+    into the package identifier for CraftBukkit and net.minecraft.server (e.g. v1_9_R2). It is incremented
+    by Spigot if there is a major breaking change in the internal server code. This is to encourage plugins
+    that interact directly with CraftBukkit/nms code to update more specifically. However, since most of
+    ProtocolLib uses reflection, and the structure of packet classes is generally static, one will have
+    to go through the code and change imports from nms/CraftBukkit._
+
+#### Ready? Let's get started!
+1. Read the [Protocol Changes](http://wiki.vg/Protocol_History). Always make sure the list is both
+complete and correct. If you're unsure, don't hesitate to ask in #mcdevs (the people who maintain wiki.vg) on [freenode.net](http://freenode.net)
+or #spigot on [irc.spi.gt](http://irc.spi.gt) ([webchat](https://irc.spi.gt/iris/?channels=spigot)).
+2. Search for usages of the now-defunct NMS package guard and change them.
+3. The class `com.comphenix.protocol.PacketType` contains a list of all the packets. If any packets were added or removed
+(or had their ID changed), make sure to update this list. If a packet was removed
+in favor of usage of another packet, instead of removing it, move it to the bottom of the list
+in its section, add a deprecation warning to it, and redirect it to the packet that replaced it.
+4. `mvn` in the root directory to build the project.
+5. If tests fail in the maven build, go through the tests to make sure you removed references to any removed packets
+and changed the NMS version guards.
+6. Increment the package version in `com.comphenix.protocol.utility.Constants`.
+7. `com.comphenix.protocol.ProtocolLibrary` contains several constants that must be updated, including the Minecraft version
+and the release date.
+8. `com.comphenix.protocol.utility.MinecraftProtocolVersion` contains a map of all the protocol version integers.
+If the protocol version has been incremented, add a new line to the map.
+9. `mvn` in root directory again. If it builds successfully, test on the appropriate version of a Spigot server. If
+the build fails, debug!

.github/ISSUE_TEMPLATE.md

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+_Follow this template except for feature requests. Use pastebin when providing /protocol dump and any relevant errors._
+
+Make sure you've done the following:
+- [ ] You're using the latest build for your server version
+- [ ] This isn't an issue caused by another plugin
+- [ ] You've checked for duplicate issues
+
+Debug paste link:
+
+Description and relevant errors: