Update README.md

Author: jvanbruegge

README.md

@@ -19,10 +19,10 @@ In this project I want to start with a simply typed lambda calculus, prove type
     -   [ ] Extend definition
     -   [ ] Prove Progress
     -   [ ] Prove Preservation
--   [ ] Extend to System F (ie introduce polymorphic variables)
-    -   [ ] Extend definition
-    -   [ ] Prove Progress
-    -   [ ] Prove Preservation
+-   [x] Extend to System F (ie introduce polymorphic variables)
+    -   [x] Extend definition
+    -   [x] Prove Progress
+    -   [x] Prove Preservation
 -   [ ] Extend System F with Coercions
     -   [ ] Extend definition
     -   [ ] Prove Progress
@@ -31,4 +31,4 @@ In this project I want to start with a simply typed lambda calculus, prove type
 
 ## Used literature
 
-Up until now, I mainly used [Software Foundations: Programming Language Foundations](https://softwarefoundations.cis.upenn.edu/current/plf-current/toc.html).
+Up until now, I mainly used [Software Foundations: Programming Language Foundations](https://softwarefoundations.cis.upenn.edu/current/plf-current/toc.html) and the [System Fc paper](https://www.microsoft.com/en-us/research/wp-content/uploads/2007/01/tldi22-sulzmann-with-appendix.pdf?from=https%3A%2F%2Fresearch.microsoft.com%2Fen-us%2Fum%2Fpeople%2Fsimonpj%2Fpapers%2Fext-f%2Ftldi22-sulzmann-with-appendix.pdf).