Add missing information about the first example in README.md; clean up example code

The first code example references a number of other pieces of code that are not mentioned or shown, making the example confusing. In particular, the definition of `Streams` and the fact that `luigi.run()` needs to be called are not mentioned. The latter is particularly confusing in the example.

Additionally, the example code is inconsistent in a number of ways. I added context managers when opening files (already done elsewhere in the file), used `str.format` rather than %s, removed trailing spaces on a number of lines, broke lines at under 80 characters, etc.