Add Prism to legal.md

Author: eregon

doc/legal/legal.md

@@ -168,6 +168,12 @@ files.
 TruffleRuby includes parts of the FFI gem (version as described in [lib/truffle/ffi/version.rb](../../lib/truffle/ffi/version.rb)).
 The FFI gem is copyright  2008-2016, Ruby FFI project contributors, and covered by the three-clause BSD licence (see `ffi.txt`).
 
+#### Prism
+
+TruffleRuby uses the [Prism](https://github.com/ruby/prism) Ruby parser
+(version as described in [src/main/c/yarp/include/prism/version.h](../../src/main/c/yarp/include/prism/version.h)),
+copyright Shopify Inc. and is available under an MIT licence (see `src/main/c/yarp/LICENSE.md`).
+
 # Java dependencies
 
 TruffleRuby has Java dependencies on these modules, which are then included in