doc: clean up license file (#3876)

GitHub gets confused by the non-standard format of the LICENSE file.
Move the extra bits into the creatively named LICENSE-extra file.

Fixes:

doc: clean up license file (#3876)

GitHub gets confused by the non-standard format of the LICENSE file.
Move the extra bits into the creatively named LICENSE-extra file.

Fixes: https://github.com/libuv/libuv/issues/3875

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