Adding pg_trgm fix to common installation problems.
Common problem on upgrading to 8.6. Adding documentation on how to fix this for Omnibus. Source already has this at https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/update/8.5-to-8.6.md#8-updates-for-postgresql-users.
Related issues at https://gitlab.com/gitlab-org/gitlab-ce/issues/14640 and https://gitlab.com/gitlab-org/gitlab-ce/issues/14530.