Mention two PHP extension dependencies in the README.

The PHP cURL extension is needed for the library itself, as can be
seen e.g. by the use of all the CURL_FOO constants. When compiling PHP
from source, this is an optional extension, so we mention it in the
README.

The test suite requires the additional JSON extension, for the
json_decode() function called throughout the suite. This is now
mentioned in the README as well.