Update README.md

Author: cyanray

README.md

@@ -10,7 +10,8 @@ A toy FEM solver written in C++20.
 - [x] Multiple elements:
   - [x] Truss
   - [x] Beam
-  - [x] Constant Strain Triangle
+  - [x] Constant Strain/Stress Triangle
+  - [x] Quad4
   - [ ] More...
 - [x] Multiple materials:
   - [x] Linear elastic