Add a file describing Google's commitments with regard to patents and…

… this project (flutter#8632)

PATENTS

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+Google hereby grants to you a perpetual, worldwide, non-exclusive,
+no-charge, royalty-free, irrevocable (except as stated in this
+section) patent license to make, have made, use, offer to sell, sell,
+import, transfer, and otherwise run, modify and propagate the contents
+of this implementation, where such license applies only to those
+patent claims, both currently owned by Google and acquired in the
+future, licensable by Google that are necessarily infringed by this
+implementation. This grant does not include claims that would be
+infringed only as a consequence of further modification of this
+implementation. If you or your agent or exclusive licensee institute
+or order or agree to the institution of patent litigation or any other
+patent enforcement activity against any entity (including a
+cross-claim or counterclaim in a lawsuit) alleging that this
+implementation constitutes direct or contributory patent infringement,
+or inducement of patent infringement, then any patent rights granted
+to you under this License for this implementation shall terminate as
+of the date such litigation is filed.