Merge pull request #164 from videlalvaro/master

adds Sammet PL history paper

plt/README.md

@@ -3,3 +3,5 @@
 * [Can Programming Be Liberated from the von Neumann Style? A Functional Style and Its Algebra of Programs](http://www.thocp.net/biographies/papers/backus_turingaward_lecture.pdf)
 
 * [Programming and Reasoning with Algebraic Effects and Dependent Types](http://eb.host.cs.st-andrews.ac.uk/drafts/effects.pdf)
+
+* [Programming Languages: History and Future](http://www.csee.umbc.edu/courses/undergraduate/CMSC331/resources/papers/sammet1972.pdf)