Elaborate about `-Cdiscovery.type=single-node`

[amotl]

About the -Cdiscovery.type=single-node, @matriv asked:

Do we explain somewhere that this is needed only for developement/testing and it shouldn't be added in prod deployments?

Originally posted by @matriv in #388 (review)

[amotl]

Hi, and thanks. We are planning to search for other spots where this is mentioned, and possibly improve the situation when applicable.

[amotl]

@mfussenegger recently stated about the vm.max_map_count=262144 requirement:

CrateDB enforces the high limit if it thinks that it is a production setup - that's based on if it is bound to a public address instead of the loopback device. Docker due to the container architecture always binds to a interface that looks public, hence it enforces the higher limit. Our examples [for single-node use] include the -Cdiscovery.type=single-node option that forces CrateDB to run in non-production mode, not enforcing any limits.