Add version 0.21 release notes
Add version 0.21 release notes

GtTmy/pyeda,cjdrake/pyeda,sschnug/pyeda,sschnug/pyeda,pombredanne/pyeda,cjdrake/pyeda,karissa/pyeda,sschnug/pyeda,cjdrake/pyeda,pombredanne/pyeda,karissa/pyeda,pombredanne/pyeda,karissa/pyeda,GtTmy/pyeda,GtTmy/pyeda