add letsencrypt to Gitea (#4189)

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

@@ -125,6 +125,11 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `REDIRECT_OTHER_PORT`: **false**: If true and `PROTOCOL` is https, redirects http requests
    on another (https) port.
 - `PORT_TO_REDIRECT`: **80**: Port used when `REDIRECT_OTHER_PORT` is true.
+- `ENABLE_LETSENCRYPT`: **false**: If enabled you must set `DOMAIN` to valid internet facing domain (ensure DNS is set and port 80 is accessible by letsencrypt validation server).
+   By using Lets Encrypt **you must consent** to their [terms of service](https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf)
+- `LETSENCRYPT_ACCEPTTOS`: **false**: This is an explicit check that you accept the terms of service for Let's Encrypt
+- `LETSENCRYPT_DIRECTORY`: **https**: Directory that Letsencrypt will use to cache information such as certs and private keys
+- `LETSENCRYPT_EMAIL`: **email@example.com**: Email used by Letsencrypt to notify about problems with issued certificates. (No default)
 
 ## Database (`database`)
 

doc/usage/https-support.md

@@ -32,6 +32,24 @@ KEY_FILE = key.pem
 ```
 To learn more about the config values, please checkout the [Config Cheat Sheet](../config-cheat-sheet#server).
 
+## Using Let's Encrypt
+
+[Let's Encrypt](https://letsencrypt.org/) is a Certificate Authority that allows you to automatically request and renew SSL/TLS certificates. In addition to starting Gitea on your configured port, to request HTTPS certificates Gitea will also need to listed on port 80, and will set up an autoredirect to HTTPS for you. Let's Encrypt will need to be able to access Gitea via the Internet to verify your ownership of the domain.
+
+By using Lets Encrypt **you must consent** to their [terms of service](https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf)
+
+```ini
+[server]
+PROTOCOL=https
+DOMAIN=git.example.com
+ENABLE_LETSENCRYPT=true
+LETSENCRYPT_ACCEPTTOS=true
+LETSENCRYPT_DIRECTORY=https
+LETSENCRYPT_EMAIL=email@example.com
+```
+
+To learn more about the config values, please checkout the [Config Cheat Sheet](../config-cheat-sheet#server).
+
 ## Using reverse proxy
 
 Setup up your reverse proxy like shown in the [reverse proxy guide](../reverse-proxies).