added comment about lat and lon projection. Related to #75

[jossefaz]

Here's the reason for this change :rocket:

• Connected to https://github.com/pelias/csv-importer/issues/75 In this issue I learned that for the moment, pelias csv importer does support only WGS projected data. In the user guide (README.md) of this project, I found usefull to add a comment on this specification.

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Here's what actually got changed :clap:

• [x] added a comment line in README.md

[missinglink]

Thanks!