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

docs/configuring-playbook-ma1sd.md

@@ -31,6 +31,8 @@ To use the [Registration](https://github.com/ma1uta/ma1sd/blob/master/docs/featu
 
 - `matrix_synapse_enable_registration` - to enable user-initiated registration in Synapse
 
+- `matrix_synapse_enable_registration_captcha` - to validate registering users using reCAPTCHA, as described in the [enabling reCAPTCHA](configuring_captcha.md) documentation.
+
 - `matrix_synapse_registrations_require_3pid` - to control the types of 3pid (`'email'`, `'msisdn'`) required by the Synapse server for registering
 
 - variables prefixed with `matrix_nginx_proxy_proxy_matrix_3pid_registration_` (e.g. `matrix_nginx_proxy_proxy_matrix_3pid_registration_enabled`) - to configure the integrated nginx webserver to send registration requests to ma1sd (instead of Synapse), so it can apply its additional functionality