Add license

Copied from MCPConfig's license:
https://github.com/MinecraftForge/MCPConfig/blob/master/LICENSE

This license retroactively applies to MCPBot data according to its commit
message:
MinecraftForge/MCPConfig@e829c04

This was also confirmed by Lex and Curle in the NeoForged Discord:
https://discord.com/channels/313125603924639766/313125603924639766/1128896543169196222
(Archive: https://github.com/LegacyModdingMC/MCPMappingsArchive/wiki/MCP-License-Discussion)

LICENSE

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+Copyright (c) 2018 Forge Development LLC
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+This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software or data.
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+Permission is granted to anyone to use this software and data for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:
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+1. The origin of this software and data must not be misrepresented; you must not claim that you wrote the original software or data. If you use this software or data in a product, an acknowledgment in the product documentation would be appreciated but is not required.
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+2. Altered versions must be plainly marked as such, and must not be misrepresented as being the original.
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+3. This notice may not be removed or altered from any distribution.