[CHLog] Updated the changlog for v6.0.4

CHANGELOG.md

-# 6.0.4 (in preparation)
+# 6.0.4
+### Features:
+ - Parallel computing framework for FEM by using PETSc, which also includes
+   - Parallel input of partitioned mesh data.
+   - Parallel output of solutions by using pvtu data format.
 
 ### Infrastructure