Add more details.

Author: kevinbackhouse

ql_demos/cpp/Facebook_Fizz_CVE-2019-3560/README.md

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-[Snapshot](https://downloads.lgtm.com/snapshots/cpp/facebook/fizz/facebookincubator_fizz_cpp-srcVersion_c69ad1baf3f04620393ebadc3eedd130b74f4023-dist_odasa-lgtm-2019-01-13-f9dca2a-universal.zip)
+# Facebook Fizz integer overflow vulnerability (CVE-2019-3560)
+
+Use [this snapshot](https://downloads.lgtm.com/snapshots/cpp/facebook/fizz/facebookincubator_fizz_cpp-srcVersion_c69ad1baf3f04620393ebadc3eedd130b74f4023-dist_odasa-lgtm-2019-01-13-f9dca2a-universal.zip) for the demo.
+
+[Fizz](https://github.com/facebookincubator/fizz) contained a remotely triggerable infinite loop. For more details about the bug, see this [blog post](https://lgtm.com/blog/facebook_fizz_CVE-2019-3560). A proof-of-concept exploit is available [here](https://github.com/Semmle/SecurityExploits/tree/446048470633bf0f8da9570d008d056dbaa28ea9/Facebook/Fizz/CVE-2019-3560).