Mention Inventory import in README

The npm command `import:data` does not trigger inventory import. To import inventories, one needs to run `import:inventory`. README may mention this perhaps with a warning that inventory import takes a long time just as in here https://github.com/commercetools/commercetools-sunrise-data/blob/master/lib/data.js#L86-L88
(Thanks go to @stmeissner for the reference to data.js.)

Start in README and compare its import instructions with the npm scripts `import:data` and `import:inventory`. Read `lib/data.js` around lines 86-88 for the existing long-running inventory import warning. Done means the README clearly explains that `import:data` does not import inventories, names `import:inventory`, and notes the expected duration.

Written by the indexing model from the issue text.