Update ssl.md to include information about required open port 80 for Let's…
Update ssl.md to include information about required open port 80 for Let's Encrypt. Needs version numbers added.
Referencing https://gitlab.com/gitlab-org/gitlab-ce/issues/43719 and empirical debugging. Not a developer / deep user so unsure about automatic behavior / thresholds for config changes between certain Gitlab versions-- marked in the text by (>version) and (<version) to denote greater / lesser than.
Thank you!