Add API load testing documentation

Author: jmarin

README.md

@@ -221,6 +221,25 @@ The Public API will run on `$(docker-machine ip):8082`
 By default, the `HDMA Platform` runs with a log level of `INFO`. This can be changed by establishing a different log level in the `HMDA_LOGLEVEL` environment variable.
 For the different logging options, see the [reference.conf](https://github.com/akka/akka/blob/master/akka-actor/src/main/resources/reference.conf#L38) default configuration file for `Akka`.
 
+#### API Load Testing
+
+A load testing scenario for the API has been built using [Gatling](https://gatling.io/). The parameters for this test can be configured in the `cluster/src/test/resources/application.conf` file.
+To run the default scenario:
+
+```shell
+$ sbt
+> project cluster
+> gatling:test
+```
+
+After the test is run, some statistics will be presented on the screen. To further study the results of the test, a report can be opened on a browser from the sbt prompt:
+
+```shell
+> gatling:lastReport
+```
+
+** NOTE: The load test requires the sample panel [file](loader/src/main/resources/inst_data_2017_dummy.csv) to be loaded before running it.
+
 #### To run the entire platform
 
 1. Ensure you have a Docker Machine with sufficient resources, as described in the [Docker](#docker) section above.