improve indexer docs

[xmonader]

readme.md

WS_ENDPOINT: is that substrate node url?
START_HEIGHT: how does that affect things?

docker-compose file

document the images used, variables and any concerns (noticed e.g singlenode with insecure flag?)

development.md

it says types.json while it uses typesBundle.json? is it just outdated info?

[DylanVerstraete]

67d140e

[sameh-farouk]

Verification Note:

This wasn't resolved

START_HEIGHT: how does that affect things?