Add changelog for 1.8.1 release.
Add changelog for 1.8.1 release.

tommyip/zulip,rishig/zulip,synicalsyntax/zulip,showell/zulip,jackrzhang/zulip,brainwane/zulip,brainwane/zulip,eeshangarg/zulip,timabbott/zulip,kou/zulip,andersk/zulip,tommyip/zulip,showell/zulip,kou/zulip,hackerkid/zulip,shubhamdhama/zulip,brainwane/zulip,shubhamdhama/zulip,zulip/zulip,brainwane/zulip,showell/zulip,rishig/zulip,shubhamdhama/zulip,andersk/zulip,showell/zulip,timabbott/zulip,punchagan/zulip,andersk/zulip,tommyip/zulip,eeshangarg/zulip,tommyip/zulip,shubhamdhama/zulip,rishig/zulip,punchagan/zulip,synicalsyntax/zulip,rishig/zulip,zulip/zulip,synicalsyntax/zulip,showell/zulip,eeshangarg/zulip,kou/zulip,brainwane/zulip,eeshangarg/zulip,rht/zulip,rishig/zulip,dhcrzf/zulip,timabbott/zulip,kou/zulip,dhcrzf/zulip,timabbott/zulip,zulip/zulip,jackrzhang/zulip,synicalsyntax/zulip,rht/zulip,hackerkid/zulip,rht/zulip,tommyip/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,timabbott/zulip,eeshangarg/zulip,rishig/zulip,shubhamdhama/zulip,hackerkid/zulip,jackrzhang/zulip,synicalsyntax/zulip,dhcrzf/zulip,synicalsyntax/zulip,timabbott/zulip,jackrzhang/zulip,jackrzhang/zulip,zulip/zulip,kou/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,dhcrzf/zulip,rht/zulip,tommyip/zulip,dhcrzf/zulip,dhcrzf/zulip,showell/zulip,zulip/zulip,jackrzhang/zulip,shubhamdhama/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,jackrzhang/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,shubhamdhama/zulip,andersk/zulip,rishig/zulip,dhcrzf/zulip,brainwane/zulip,kou/zulip,tommyip/zulip,kou/zulip,punchagan/zulip,showell/zulip,timabbott/zulip,andersk/zulip,synicalsyntax/zulip,zulip/zulip,punchagan/zulip