Create src/README.md

[lautarodragan]

• Explain architecture
• Explain that each subfolder has its own README explaining the inner workings in more detail
• Update main README.md with an Architecture section linking to this file
• For src/extensions, link to sugarjs for arguments in favor of it and explanations. Also possibly mention how we may choose to drop this style in the future if we adopt Ramda, since, for example, myArray.filter(evenNumbers).toObject() reads better than arrayToObject(myArray.filter(evenNumbers)) for the same reasons listed in the pipeline operator proposal, but something like pipe(filter(isEven), arrayToObject)(myArray) could work.