Add variables for reCAPTCHA validation

docs/configuring-captcha.md

@@ -0,0 +1,24 @@
+(Adapted from the [upstream project](https://github.com/matrix-org/synapse/blob/develop/docs/CAPTCHA_SETUP.md))
+
+# Overview
+Captcha can be enabled for this home server. This file explains how to do that.
+The captcha mechanism used is Google's ReCaptcha. This requires API keys from Google.
+
+## Getting keys
+
+Requires a site/secret key pair from:
+
+<https://developers.google.com/recaptcha/>
+
+Must be a reCAPTCHA v2 key using the "I'm not a robot" Checkbox option
+
+## Setting ReCaptcha Keys
+
+Once registered as above, set the following values:
+
+    matrix_synapse_recaptcha_public_key: 'YOUR_SITE_KEY'
+    matrix_synapse_recaptcha_private_key: 'YOUR_SECRET_KEY'
+
+In addition, you MUST enable captchas via:
+
+    matrix_synapse_enable_registration_captcha: true