Update USER_GUIDE file

USERS_GUIDE.md

@@ -72,6 +72,7 @@ Using *ELPA* with the latest API is done in the following steps
       real/complex matrices
     - "cholesky" does a cholesky factorization for a single/double real/complex matrix
     - "invert_triangular" inverts a single/double real/complex triangular matrix
+    - "solve_tridiagonal" solves the single/double eigenvalue problem for a real tridiagonal matrix
 
 - if the ELPA object is not needed any more call ELPA-type function destroy