Add helpful comments to generated .env file

[t-kelly]

What are you trying to accomplish with this PR?

Fixes #398. Generates the following .env file:

# The myshopify.com URL to your Shopify store
SLATE_STORE=

# The API password generated from a Private App
SLATE_PASSWORD=

# The ID of the theme you wish to upload files too
SLATE_THEME_ID=

# A list of file patterns to ignore, with each list item seperated by ':'
SLATE_IGNORE_FILES=

[lock]

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