add description to a certificate

infotro/SDi/labs/SSL/README.md

@@ -4,22 +4,26 @@ Lab OpenSSL & Certificates
 
 ----------
 
-# Create an RSA key
+A certificate is a digital document that certifies the ownership of a public key by the named subject of the certificate.
+The certificate is signed by a certificate authority (CA) that is trusted by the parties who trust the certificate.
+The certificate authority is responsible for verifying the identity of the certificate holder and signing the certificate.
+
+## Create an RSA key
 ```bash
 openssl genrsa -out <name>.key 2048
 ```
 
-# Self-sign root certificate 
+## Self-sign root certificate 
 ```bash
 openssl req -new -x509 -days 365 -key ca.key -config ca.cnf -out ca.crt
 ```
 
-# Create a request
+## Create a request
 ```bash
 openssl req -new -sha256 -key <my_site>.key -config <my_site>.cnf -out <my_site>.csr
 ```
 
-# Sign a request with root certificate
+## Sign a request with root certificate
 ```bash
 openssl ca -config ca_ssl.cnf -in <my_site>.csr -out <my_site>.crt
 ```