make note about docker memory requirements

[hartman]

On initial install my lerna bootstrap was failing. This was because gyp was compiling mapnik and it hit the 2GB default memory limit of my docker environment. I suspect actual usage of tilerator also requires considerable memory, so we should probably write this down.

[thesocialdev]

Sounds good, for now I'll write down a warning in the readme regarding memory usage. But will leave it open until there is a proper benchmark. I haven't noticed that because I don't limit my docker setup.