CIPHERS.md: Mention the options used to set TLS 1.3 ciphers

# Ciphers

With curl's options `CURLOPT_SSL_CIPHER_LIST` and `--ciphers` users can

control which ciphers to consider when negotiating TLS connections.

With curl's options

[`CURLOPT_SSL_CIPHER_LIST`](https://curl.haxx.se/libcurl/c/CURLOPT_SSL_CIPHER_LIST.html)

and

[`--ciphers`](https://curl.haxx.se/docs/manpage.html#--ciphers)

users can control which ciphers to consider when negotiating TLS connections.

TLS 1.3 ciphers are supported since curl 7.61 with options

[`CURLOPT_TLS13_CIPHERS`](https://curl.haxx.se/libcurl/c/CURLOPT_TLS13_CIPHERS.html)

and

[`--tls13-ciphers`](https://curl.haxx.se/docs/manpage.html#--tls13-ciphers)

.

The names of the known ciphers differ depending on which TLS backend that

libcurl was built to use. This is an attempt to list known cipher names.

### TLS 1.3 cipher suites

(Note: the TLS 1.3 cipher suites are set with a separate option.)

(Note these ciphers are set with `CURLOPT_TLS13_CIPHERS` and `--tls13-ciphers`)

`TLS13-AES-256-GCM-SHA384`

`TLS13-CHACHA20-POLY1305-SHA256`