Update README.md

Add mention of CURLOPT_USERPWD , this is needed to make standard https://username:pwd@example.com  calls.

Author: robe2

README.md

@@ -278,6 +278,7 @@ Select [CURL options](https://curl.se/libcurl/c/curl_easy_setopt.html) are avail
 * [CURLOPT_CONNECTTIMEOUT](https://curl.se/libcurl/c/CURLOPT_CONNECTTIMEOUT.html)
 * [CURLOPT_CONNECTTIMEOUT_MS](https://curl.se/libcurl/c/CURLOPT_CONNECTTIMEOUT_MS.html)
 * [CURLOPT_USERAGENT](https://curl.se/libcurl/c/CURLOPT_USERAGENT.html)
+* [CURLOPT_USERPWD](https://curl.se/libcurl/c/CURLOPT_USERPWD.html)
 
 For example,