Outline all person identifiers and their validation

ynr/apps/people/README.md

@@ -10,3 +10,25 @@ standing in.
 Thing like their political party should be stored on the _candidacy_.
 This is currently represented as a `Membership` in the `popolo` app.
 
+# Person Identifiers
+
+`PersonIdentifiers` are used to store contact information and online presence for a candidate. Users can add/edit these identifiers in the person update form. The following fields are available in the `PersonIdentifier` model with the corresponding form labels and accepted formats:
+```
+email = "Email": Accepts a valid email address; raises an exception if the email is submitted with an invalid format. The email address must be unique.
+facebook_page_url = "Facebook Page": Accepts a valid URL but does not currently validate the domain.
+facebook_personal_url = "Facebook Personal": Same as above. 
+homepage_url = "Homepage": Accepts a valid URL
+blog_url = "Blog": Accepts a valid URL
+linkedin_url = "Linkedin": Accepts a valid URL; does not currently validate the domain or the format of the username.
+party_ppc_page_url = "Party Candidate Page": Accepts a valid URL
+twitter_username = "Twitter": Accepts a Twitter username and returns the full Twitter URL.No longer validates the username actually exists.
+mastodon_username = "Mastodon": Accepts and returns a valid Mastodon URL and validates the domain and username.
+wikipedia_url = "Wikipedia": Accepts a valid URL; does not currently validate the domain.
+wikidata_id = "Wikidata": Accepts a valid Wikidata ID
+youtube_profile = "YouTube Profile": Accepts a valid URL; does not currently validate the domain.
+instagram_url = "Instagram Profile": Accepts and returns a valid Instagram URL and validates the domain and format of the username.
+blue_sky_url = "Bluesky URL": Accepts and returns a valid URL
+threads_url = "Threads URL": Accepts and returns a valid URL
+tiktok_url = "TikTok URL": Accepts and returns a valid URL
+other_url = "Other URL": Accepts and returns a valid URL
+```