opam file: add synopsis

8 jobs for master in 8 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #30816
fp
build-coq.8.7.0

00:02:35

passed #30815
fp
build-coq.8.7.1

00:02:43

passed #30814
fp
build-coq.8.7.2

00:02:41

passed #30813
fp-timing
build-coq.8.8.0

00:02:36

passed #30812
fp
build-coq.8.8.1

00:02:52

passed #30811
fp
build-coq.8.8.2

00:02:52

passed #30810
fp-timing
build-coq.8.9.0

00:03:05

passed #30809
fp
build-coq.dev

00:07:53