[SSL_MD] ADDED fullchain certificate usage in ssl.md

This change in meant to help let's encrypt and derived services users to get https apps working.

Author: OBEsensia

docs/guides/ssl.md

@@ -5,10 +5,13 @@ Crow supports HTTPS though SSL or TLS.<br><br>
     When mentioning SSL in this documentation, it is often a reference to openSSL, which includes TLS.<br><br>
 
 
-To enable SSL, first your application needs to define either a `.crt` and `.key` files, or a `.pem` file.
+To enable SSL, first your application needs to define either a `.crt` and `.key` files, or a `.pem` file. You can create them using openssl locally or use CA based ones
 Once you have your files, you can add them to your app like this:<br>
 `#!cpp app.ssl_file("/path/to/cert.crt", "/path/to/keyfile.key")` or `#!cpp app.ssl_file("/path/to/pem_file.pem")`. Please note that this method can be part of the app method chain, which means it can be followed by `.run()` or any other method.<br><br>
 
+If you are using fullchain certificate files such as the ones issued using let's encrypt/certbot/acme.sh, the CA part should be provided to crow and processed accordingly.<br>
+To do so, you should not use `#!cpp app.ssl_file("/path/to/cert.crt", "/path/to/keyfile.key")` but rather `#!cpp app.ssl_chainfile("/path/to/fullchain.cer", "/path/to/keyfile.key")` or your application will retrun `[ERROR   ] Could not start adaptor: tlsv1 alert unknown ca (SSL routines)` on request and client side will get a similar error `#!bash ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1007)`.<br><br>
+
 You also need to define `CROW_ENABLE_SSL` in your compiler definitions (`g++ main.cpp -DCROW_ENABLE_SSL` for example) or `set(CROW_ENABLE_SSL ON)` in `CMakeLists.txt`.
 
 You can also set your own SSL context (by using `asio::ssl::context ctx`) and then applying it via the `#!cpp app.ssl(ctx)` method.<br><br>