update readme with clarifying socrata api documentation

[ryan-hall]

Adding a note describing differences in writing to Socrata datasets from RSocrata/SODA compared to the Data Management API. This is to help prevent confusion when automating data updates to Socrata that should make use of transforms applied on the platform during ingress.

[tomschenkjr]

Ryan-

Thank you for raising this and for the explanation. It looks like this description is in the README.md file. Do you think it would be useful to also include in the R help files, e.g., ?write.socrata as well?

Not pertinent to this particular pull request, but relatedly, would it be valuable to shift write.socrata to support the Socrata Data Management API? It appears the "old" way is not compatible with SDM methods. But, do you know if SDM actions are backward compatible with those who originally created it through the old method? We may need to think how to arrange the various write methods.

Finally, thank you for signing the CLA. It appears there is some issues with passing unit tests. But, is unrelated to your contributions so we can solve that at another time.

[ryan-hall]

excellent idea to include with ?write.socrata as well.

re: shifting to SDM, wholly agree on some thought needed for the various write methods. I plan on working on that this quarter and would appreciate any and all input.