Added Snap! Reference Manual (#3453)

* Added Snap! Reference Manual

* Fix link

* Fixed build error

free-programming-books.md

@@ -196,6 +196,7 @@
 * [Sed](#sed)
 * [Self](#self)
 * [Smalltalk](#smalltalk)
+* [Snap](#snap)
 * [Spark](#spark)
 * [SQL (implementation agnostic)](#sql-implementation-agnostic)
 * [SQL Server](#sql-server)
@@ -2478,6 +2479,11 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Stef's Free Online Smalltalk Books](http://stephane.ducasse.free.fr/FreeBooks.html) (meta-list)
 
 
+### Snap
+
+* [Snap! Reference Manual](https://snap.berkeley.edu/snapsource/help/SnapManual.pdf) - B. Harvey, J. Mönig (PDF)
+
+
 ### Spark
 
 * [Databricks Spark Knowledge Base](https://www.gitbook.com/book/databricks/databricks-spark-knowledge-base/details)