Update CHANGELOG with some highlights from the last 18 months!!
Update CHANGELOG with some highlights from the last 18 months!!

jchodera/cclib,ghutchis/cclib,ATenderholt/cclib,gaursagar/cclib,cclib/cclib,ben-albrecht/cclib,Schamnad/cclib,berquist/cclib,Schamnad/cclib,langner/cclib,langner/cclib,andersx/cclib,jchodera/cclib,andersx/cclib,berquist/cclib,berquist/cclib,ben-albrecht/cclib,Clyde-fare/cclib,cclib/cclib,ATenderholt/cclib,ghutchis/cclib,Clyde-fare/cclib,gaursagar/cclib,langner/cclib,cclib/cclib