Update README to reflect deprecation of cURL.

Author: Trevor Miranda

README.md

@@ -530,9 +530,16 @@ If you need a more complex setup, rustup supports the convention used by
 the __curl__ program, documented in the ENVIRONMENT section of
 [its manual page][curlman].
 
-Note that some versions of `libcurl` apparently require you to drop the 
-`http://` or `https://` prefix in environment variables. For example, 
-`export http_proxy=proxy.example.com:1080` (and likewise for HTTPS). 
+The use of `curl` is presently **deprecated**, however it can still be used by
+providing the `RUSTUP_USE_CURL` environment variable, for example:
+
+```
+RUSTUP_USE_CURL=1 rustup update
+```
+
+Note that some versions of `libcurl` apparently require you to drop the
+`http://` or `https://` prefix in environment variables. For example,
+`export http_proxy=proxy.example.com:1080` (and likewise for HTTPS).
 If you are getting an SSL `unknown protocol` error from `rustup` via `libcurl`
 but the command-line `curl` command works fine, this may be the problem.