Readme: properly document OPENSSL_SEARCH_PATH

Since 28a3114c the documentation regarding openssl search paths we pass
to cmake is out of date.

Properly document the current behaviour.
Signed-off-by: Marcus Hoffmann <m.hoffmann@cartelsol.com>

README.md

@@ -87,8 +87,7 @@ Variable | Default Value | Description
 ------------ | ------------- | -------------
 PAHO_BUILD_STATIC | FALSE | Build a static version of the libraries
 PAHO_WITH_SSL | FALSE | Flag that defines whether to build ssl-enabled binaries too. 
-OPENSSL_INC_SEARCH_PATH | "" (system default) | Directory containing OpenSSL includes
-OPENSSL_LIB_SEARCH_PATH | "" (system default) | Directory containing OpenSSL libraries
+OPENSSL_SEARCH_PATH | "" (system default) | Directory containing your OpenSSL installation (i.e. `/usr/local` when headers are in `/usr/local/include` and libraries are in `/usr/local/lib`)
 PAHO_BUILD_DOCUMENTATION | FALSE | Create and install the HTML based API documentation (requires Doxygen)
 PAHO_BUILD_SAMPLES | FALSE | Build sample programs
 MQTT_TEST_BROKER | tcp://localhost:1883 | MQTT connection URL for a broker to use during test execution