Update README.md

[stephen-soltesz]

This change updates the gardener README with a short overview of this component in the etl data pipeline, and provides example commands for local development, running unit tests, as well as removing (believed) obsolete operational steps (https://github.com/m-lab/etl-gardener/issues/373)

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This change is 

[stephen-soltesz]

FYI: @SaiedKazemi you might enjoy this change. :)

[SaiedKazemi]

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README.md, line 11 at r1 (raw file):

## Overview

The v2 date pipeline depends on the gardener for daily and historical

Should date be data?

[SaiedKazemi]

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README.md, line 21 at r1 (raw file):

For both of these modes, gardener issues Jobs (dates) to parsers that request
them. The parsers will enuemerate all files for that date and parse each, and
reports status updates to the gardener for the Job date until all are complete.

Should reports be report?

[SaiedKazemi]

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README.md, line 39 at r1 (raw file):

Gardener maintains a status page on a separate status port, that summarizes
recent jobs, current state, and any errors. Jobs transition through a standard

Nit - suggesting s/a/the following/

[SaiedKazemi]

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README.md, line 43 at r1 (raw file):

* Parsing - gardener has issued a job and a parser is working on it.
* postProcessing - the parser completed parsing a job.

Nit - suggesting s/post/Post/

[SaiedKazemi]

@stephen-soltesz Yes, I enjoyed the change :) Thanks a lot. Made a few nitpick suggestions.

[coveralls]

Coverage remained the same at 61.144% when pulling a816b02c06afb4e81a86dd3e07e176ba051204bf on update-readme into f2a74a18b639ba05f8f65f9fa3574f1adcf15400 on master.