document heap size configuration (elastic#45625)

CONTRIBUTING.md

@@ -173,6 +173,10 @@ yarn kbn bootstrap
 
 (You can also run `yarn kbn` to see the other available commands. For more info about this tool, see https://github.com/elastic/kibana/tree/master/packages/kbn-pm.)
 
+#### Increase node.js heap size
+
+Kibana is a big project and for some commands it can happen that the process hits the default heap limit and crashes with an out-of-memory error. If you run into this problem, you can increase maximum heap size by setting the `--max_old_space_size` option on the command line. To set the limit for all commands, simply add the following line to your shell config: `export NODE_OPTIONS="--max_old_space_size=2048"`.
+
 ### Running Elasticsearch Locally
 
 There are a few options when it comes to running Elasticsearch locally: