Add Yara license to src/third_party.

third_party/yara/README.chromium

Index: third_party/yara/README.chromium
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+Name: Yara
+URL: https://github.com/virustotal/yara
+Version: 74734418a256c5304ccaf1d322c57e305ff75362
+Date: (OPTIONAL if version is supplied) The date that the package was updated
+Revision: (OPTIONAL if version is supplied) The current revision of the package
+License: BSD 3-Clause
+License File: src/COPYING
+Security Critical: Yes
+
+Description: YARA is a tool aimed at (but not limited to) helping malware
+researchers to identify and classify malware samples. With YARA you can create
+descriptions of malware families (or whatever you want to describe) based on
+textual or binary patterns. Each description, a.k.a rule, consists of a set of
+strings and a boolean expression which determine its logic.
+
+Yara is shipped with the chrome_cleaner component downloaded by Google Chrome
+and the software_reporter_tool component fetched by the component updater. It is
+not linked directly into Chromium binaries.
+
+Local Modifications:
+None