Include license file in alloc-stdlib crate

[ankane]

Hi, this symlinks the license file so it's included in the published alloc-stdlib crate (could also copy it if it's that better).

Fixes #9

[CLAassistant]

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[ankane]

I'm not really interested in signing a CLA, but hopefully someone at Dropbox can apply this.

cd alloc-stdlib
ln -s ../LICENSE .