WIP - add documentation about what the warnings/errors mean and what …

…you can do about them.

docs/link-status-messages.md

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+# Link Status Messages
+
+The link checker runs overnight with the list of current links, and the outcome of that check is displayed in various places: the Broken Links tab, the Councils tab (council homepages only), in the individual council pages and in the individual service pages. They are in three groups.
+
+# Good
+
+All links that are marked Good have been visited by the link checker without a problem. No action is required.
+
+# Warning
+
+Warning links highlight pages that cannot be properly checked, and come in the following types.
+
+* Note: Security problem
+* Note: Page blocks robots - this can mean the page is behind a CDN (such as Cloudflare) which treats the GOV.UK webcrawler as a potential attack and
+  prompts it with a captcha to continue (the crawler cannot interact with the captcha).
+
+# Error
+
+* Broken: Website unavailable - this means the link returns a 404. SOLUTION: find the correct link and edit it.
+* missing - this means the link is empty. SOLUTION: find the correct link and edit it.