Document how element tags work (#923)

Author: deviantintegral

README.md

@@ -29,6 +29,8 @@ Images included:
 ## Running the images
 :exclamation: When executing `docker run` for an image with Chrome or Firefox please either mount `-v /dev/shm:/dev/shm` or use the flag `--shm-size=2g` to use the host's shared memory.
 
+:exclamation: In general, use a tag with an element suffix to pin a specific browser version. See [Tagging Conventions](https://github.com/SeleniumHQ/docker-selenium/wiki/Tagging-Convention) for details.
+
 Chrome
 ``` bash
 $ docker run -d -p 4444:4444 -v /dev/shm:/dev/shm selenium/standalone-chrome:3.141.59-palladium