Clarify relationship with elm-units etc.

[ianmackenzie]

Do a pass through the docs and point out where types/modules come from other packages; may also be worth adding a section to the README like "Relationship to other packages" or similar.

[ianmackenzie]

See also ianmackenzie/elm-geometry-svg#12, ianmackenzie/elm-3d-scene#28

[dimpase]

Note that the "Using the package" instructions in https://package.elm-lang.org/packages/ianmackenzie/elm-geometry/latest/ don't work out of the box, as elm-units is not in direct dependencies after elm install ianmackenzie/elm-geometry mentioned just before that section.

A clarification would be great, as this is perhaps the first code using elm-geometry people might try (and get an error due to a missing dependency).

[ianmackenzie]

Good point! I've updated the README so that at least that is clear, but there probably still needs to be a more thorough review at some point.