Update Patent Grant.

Summary:
https://code.facebook.com/posts/1639473982937255/updating-our-open-source-patent-grant/

This has been done by other FB's open source projects already:
facebook/react@b8ba8c8
https://github.com/facebook/osquery/blob/master/PATENTS

Test Plan: make check

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: jamesgpearce, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D36879

PATENTS

@@ -1,23 +1,33 @@
-Additional Grant of Patent Rights
+Additional Grant of Patent Rights Version 2
 
-“Software” means the rocksdb software distributed by Facebook, Inc.
+"Software" means the RocksDB software distributed by Facebook, Inc.
 
-Facebook hereby grants you a perpetual, worldwide, royalty-free,
-non-exclusive, irrevocable (subject to the termination provision below)
-license under any rights in any patent claims owned by Facebook, to make,
-have made, use, sell, offer to sell, import, and otherwise transfer the
-Software. For avoidance of doubt, no license is granted under Facebook’s
-rights in any patent claims that are infringed by (i) modifications to the
-Software made by you or a third party, or (ii) the Software in combination
-with any software or other technology provided by you or a third party.
+Facebook, Inc. ("Facebook") hereby grants to each recipient of the Software
+("you") a perpetual, worldwide, royalty-free, non-exclusive, irrevocable
+(subject to the termination provision below) license under any Necessary
+Claims, to make, have made, use, sell, offer to sell, import, and otherwise
+transfer the Software. For avoidance of doubt, no license is granted under
+Facebook’s rights in any patent claims that are infringed by (i) modifications
+to the Software made by you or any third party or (ii) the Software in
+combination with any software or other technology.
 
-The license granted hereunder will terminate, automatically and without
-notice, for anyone that makes any claim (including by filing any lawsuit,
-assertion or other action) alleging (a) direct, indirect, or contributory
-infringement or inducement to infringe any patent: (i) by Facebook or any
-of its subsidiaries or affiliates, whether or not such claim is related
-to the Software, (ii) by any party if such claim arises in whole or in
-part from any software, product or service of Facebook or any of its
-subsidiaries or affiliates, whether or not such claim is related to the
-Software, or (iii) by any party relating to the Software; or (b) that
-any right in any patent claim of Facebook is invalid or unenforceable.
+The license granted hereunder will terminate, automatically and without notice,
+if you (or any of your subsidiaries, corporate affiliates or agents) initiate
+directly or indirectly, or take a direct financial interest in, any Patent
+Assertion: (i) against Facebook or any of its subsidiaries or corporate
+affiliates, (ii) against any party if such Patent Assertion arises in whole or
+in part from any software, technology, product or service of Facebook or any of
+its subsidiaries or corporate affiliates, or (iii) against any party relating
+to the Software. Notwithstanding the foregoing, if Facebook or any of its
+subsidiaries or corporate affiliates files a lawsuit alleging patent
+infringement against you in the first instance, and you respond by filing a
+patent infringement counterclaim in that lawsuit against that party that is
+unrelated to the Software, the license granted hereunder will not terminate
+under section (i) of this paragraph due to such counterclaim.
+
+A "Necessary Claim" is a claim of a patent owned by Facebook that is
+necessarily infringed by the Software standing alone.
+
+A "Patent Assertion" is any lawsuit or other action alleging direct, indirect,
+or contributory infringement or inducement to infringe any patent, including a
+cross-claim or counterclaim.