Improve documentation about headers

[kelle]

Based on discussion in #617 , there is a need for more documentation about headers.

I propose a Header section be added to Working with Spectrum1Ds.

Something like:

Headers

There is currently not universally standard behavior for the handling of header information. In particular, header data is not guaranteed to be preserved or updated. Users should take care with monitoring any important header keywords.

A common practice is to store the primary header in meta['header'] rather than in meta directly.

[a couple examples..]