Add Spark SL as third party dep

Summary: We already have [Pre-release tag](https://github.com/facebook/igl/releases/tag/SparkSL) now we can link to it.

Reviewed By: corporateshark

Differential Revision: D47473998

fbshipit-source-id: be73db1ee087d13dbfe970771177ece5139cf917

LICENSE.md

@@ -74,6 +74,9 @@ https://github.com/leetal/ios-cmake/blob/master/LICENSE.md
 Meshoptimizer
 https://github.com/zeux/meshoptimizer/blob/master/LICENSE.md
 
+Spark SL
+https://github.com/facebook/igl/releases/download/SparkSL/SparkSL.LICENSE
+
 stb
 https://github.com/nothings/stb/blob/master/LICENSE
 

README.md

@@ -111,4 +111,4 @@ cmake --build .
 ## License
 
 IGL is released under the MIT license, see [LICENSE.md](./LICENSE.md) for the full text as well as third-party library
-acknowledgements. SparkSL Compiler is released under the SparkSL Compiler License, see LICENSE.md for full text.
+acknowledgements. SparkSL Compiler is released under the SparkSL Compiler License, see [LICENSE](https://github.com/facebook/igl/releases/download/SparkSL/SparkSL.LICENSE) for full text.

third-party/bootstrap-deps.json

@@ -154,5 +154,13 @@
         "url": "https://github.com/leetal/ios-cmake.git",
         "revision": "04d91f6675dabb3c97df346a32f6184b0a7ef845"
     }
+},
+{
+    "name": "spark-sl",
+    "source": {
+        "type": "archive",
+        "url": "https://github.com/facebook/igl/releases/download/SparkSL/SparkSL_Compiler-v0.0.1.zip",
+        "sha1": "d636517b5fd3103f7aeca2ed6d4c9ad3e2e97964"
+    }
 }
 ]