docs: Explicitly state that express instrumentation does not export s…

…pans without http instrumentation (open-telemetry#472)

As discussed in the meeting today, I think making this point clear helps enduser to understand what they _might_ be doing wrong if no spans are exported.

Co-authored-by: Valentin Marchaud <contact@vmarchaud.fr>

plugins/node/opentelemetry-instrumentation-express/README.md

@@ -12,7 +12,7 @@ For automatic instrumentation see the
 
 ## Installation
 
-This instrumentation relies on HTTP calls to also be instrumented. Make sure you install and enable both.
+This instrumentation relies on HTTP calls to also be instrumented. Make sure you install and enable both, otherwise you will not see any spans being exported from the instrumentation.
 
 ```bash
 npm install --save @opentelemetry/instrumentation-http @opentelemetry/instrumentation-express