add docs for REQUIRE_EXTERNAL_REGISTRATION_CAPTCHA and REQUIRE_EXTERNAL_REGISTRATION_PASSWORD

Author: AJ ONeal

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -202,6 +202,9 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
    Requires `Mailer` to be enabled.
 - `DISABLE_REGISTRATION`: **false**: Disable registration, after which only admin can create
    accounts for users.
+- `REQUIRE_EXTERNAL_REGISTRATION_PASSWORD`: **false**: Enable this to force externally created
+   accounts (via GitHub, OpenID Connect, etc) to create a password. Warning: enabling this will
+   decrease security, so you should only enable it if you know what you're doing.
 - `REQUIRE_SIGNIN_VIEW`: **false**: Enable this to force users to log in to view any page.
 - `ENABLE_NOTIFY_MAIL`: **false**: Enable this to send e-mail to watchers of a repository when
    something happens, like creating issues. Requires `Mailer` to be enabled.
@@ -211,6 +214,8 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `ENABLE_REVERSE_PROXY_EMAIL`: **false**: Enable this to allow to auto-registration with a
    provided email rather than a generated email.
 - `ENABLE_CAPTCHA`: **false**: Enable this to use captcha validation for registration.
+- `REQUIRE_EXTERNAL_REGISTRATION_CAPTCHA`: **false**: Enable this to force captcha validation
+   even for External Accounts (i.e. GitHub, OpenID Connect, etc).
 - `CAPTCHA_TYPE`: **image**: \[image, recaptcha\]
 - `RECAPTCHA_SECRET`: **""**: Go to https://www.google.com/recaptcha/admin to get a secret for recaptcha.
 - `RECAPTCHA_SITEKEY`: **""**: Go to https://www.google.com/recaptcha/admin to get a sitekey for recaptcha.