new publication about crystal (#2238)

free-programming-books.md

@@ -26,6 +26,7 @@
 * [ColdFusion](#coldfusion)
 * [Cool](#cool)
 * [Coq](#coq)
+* [Crystal](#crystal)
 * [CUDA](#cuda)
 * [D](#d)
 * [Dart](#dart)
@@ -936,6 +937,10 @@
 * [Software Foundations](http://www.cis.upenn.edu/~bcpierce/sf/)
 
 
+### Crystal
+
+*[Crystal for Rubyists.](http://www.crystalforrubyists.com)
+
 ### CUDA
 
 * [OpenCL Programming Guide for CUDA Architecture](http://www.nvidia.com/content/cudazone/download/OpenCL/NVIDIA_OpenCL_ProgrammingGuide.pdf) (PDF)