Add release notes project URL

Background info at
  https://github.com/pypa/warehouse/pull/7882#issue-412444446
13 jobs for relnotes-project-url in 3 minutes and 26 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #539062043
docs

00:01:30

passed #539062045
dogfood

00:01:15

passed #539062040
linters

00:01:59

passed #539062042
pre-commit

00:01:27

passed #539062020
pypy2

00:02:02

passed #539062023
pypy3

00:02:20

passed #539062027
python2

00:01:16

passed #539062030
python34

00:01:22

passed #539062031
python35

00:02:04

passed #539062034
python36

00:01:16

passed #539062036
python37

00:01:17

passed #539062039
python38

00:01:18

 
  Build
passed #539062048
build

00:01:05