Add EC key generation paragraph in doc/HOWTO/keys.txt

Seems this documentation is not dead, so add this missing part
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4037)

doc/HOWTO/keys.txt

@@ -27,12 +27,6 @@ With this variant, you will be prompted for a protecting password.  If
 you don't want your key to be protected by a password, remove the flag
 '-des3' from the command line above.
 
-    NOTE: if you intend to use the key together with a server
-    certificate, it may be a good thing to avoid protecting it
-    with a password, since that would mean someone would have to
-    type in the password every time the server needs to access
-    the key.
-
 The number 2048 is the size of the key, in bits.  Today, 2048 or
 higher is recommended for RSA keys, as fewer amount of bits is
 consider insecure or to be insecure pretty soon.
@@ -62,11 +56,50 @@ With this variant, you will be prompted for a protecting password.  If
 you don't want your key to be protected by a password, remove the flag
 '-des3' from the command line above.
 
-    NOTE: if you intend to use the key together with a server
-    certificate, it may be a good thing to avoid protecting it
-    with a password, since that would mean someone would have to
-    type in the password every time the server needs to access
-    the key.
 
--- 
-Richard Levitte
+4. To generate an EC key
+
+An EC key can be used both for key agreement (ECDH) and signing (ECDSA).
+
+Generating a key for ECC is similar to generating a DSA key. These are
+two-step processes. First, you have to get the EC parameters from which
+the key will be generated:
+
+  openssl ecparam -name prime256v1 -out prime256v1.pem
+
+The prime256v1, or NIST P-256, which stands for 'X9.62/SECG curve over
+a 256-bit prime field', is the name of an elliptic curve which generates the
+parameters. You can use the following command to list all supported curves:
+
+  openssl ecparam -list_curves
+
+When that is done, you can generate a key using the created parameters (several
+keys can be produced from the same parameters):
+
+  openssl genpkey -des3 -paramfile prime256v1.pem -out private.key
+
+With this variant, you will be prompted for a password to protect your key.
+If you don't want your key to be protected by a password, remove the flag
+'-des3' from the command line above.
+
+You can also directly generate the key in one step:
+
+  openssl ecparam -genkey -name prime256v1 -out private.key
+
+or
+
+  openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256
+
+
+5. NOTE
+
+If you intend to use the key together with a server certificate,
+it may be reasonable to avoid protecting it with a password, since
+otherwise someone would have to type in the password every time the
+server needs to access the key.
+
+For X25519, it's treated as a distinct algorithm but not as one of
+the curves listed with 'ecparam -list_curves' option. You can use
+the following command to generate an X25519 key:
+
+  openssl genpkey -algorithm X25519 -out xkey.pem