docs: have multiple points in 1 request

[ajnisbet]

Thanks for highlighting this! I'd like to keep the README pretty simple in favour of the complete documentation website, so won't merge the code example, but did just add a reference to multi-point support.

https://github.com/ajnisbet/opentopodata/commit/fc1114606adf2dfe780500e7095afaebc00116d2