Update README.md

add notice to MacOS Docker user, if docker  memory limit is less than 2GiB, build process may fail when run ./node_modules/.bin/grunt build

README.md

@@ -90,6 +90,8 @@ Choose this option to build on platforms other than linux/amd64 and/or not have
 
 The resulting image will be tagged as `grafana/grafana:dev`
 
+Notice: If you are using Docker for MacOS, be sure to let limit of Memory bigger than 2 GiB, otherwize you may faild at `grunt build`
+
 ### Dev config
 
 Create a custom.ini in the conf directory to override default configuration options.