Reorganized and expanded the CHANGELOG -- this should now have everything major since version 3.3.
Reorganized and expanded the CHANGELOG -- this should now have everything major
since version 3.3.

** Feel free to edit and please verify that I haven't missed something! **

[ci skip

mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem