Update README to reflect deprecation of cURL.
@@ -530,6 +530,13 @@ 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].
+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).
