[CI-SKIP] Clarify project licensing

* THIS IS NOT A RELICENSE *

This commit adds explicit clarification as to the state of this
project's licensing as it has always been. Nothing has changed here.

It also adds a note about how specific authors have chosen to release
their work under a more permissive license. Any past contributors who
want their name added to this list should submit a pull request here.

We will not be adding anyone's names to this list ourselves.

LICENSE.md

@@ -0,0 +1,19 @@
+Paper inherits its licensing from upstream projects.
+
+As such, Paper is licensed under the
+[GNU General Public License version 3](licenses/GPL.md); as it inherits it from Spigot,
+who in turn inherits it from the original Bukkit and Craftbukkit projects.
+
+Any author who is _not_ listed below should be presumed to have released their work
+under the original [GPL](licenses/GPL.md) license.
+
+In the interest of promoting a better Minecraft platform for everyone, contributors
+may choose to release their code under the more permissive [MIT License](licenses/MIT.md).
+
+The authors listed below have chosen to release their code under that more permissive
+[MIT License](licenses/MIT.md). Any contributor who wants their name added below
+should submit a pull request to this project to add their name.
+
+```text
+Zach Brown <[email protected]>
+```