Update to the guide.md file.

Changed it to 2nd person, present tense. Replaced “may” with “might” or
“can” in context. Added bullet lists where appropriate. Removed some
marketing speak (“easily,” “simply”) and some inadvertently
condescending language. This page still needs a substantial rewrite,
reversing the order of items so that we start with entities and work up
to datasets.
Adding Creating Dataset Views section

Includes documentation for the Views API and using Datasets in
MapReduce.