Update snippet-template.ly

Make the metainfo section more elaborate.
It looks quite long but I think it's better to have it 'feature-rich'
Update snippet-template.ly

Incorporate comments on 9019ba347d86ef9f9fcee07b0725683005441fda
changed 'title' to 'snippet-title' to prevent Lily from printing it.

...because if some fields from the header are actually printed, 'preview'
option (for cropping output) doesn't work as we want it to.
'we'd rather have undocumented snippets than no snippets'
template: make shorter

I don't want to overwhelm contributors.  Finding a good balance
between putting enough information and keeping things brief is
admittedly hard.
template: reword
template: remove 'category'

I'd say that:
- placing a snippet in a directory should provide enough categorization
- if we really wanted to write the category in the file, it should indeed
  be one of a given list.  But this could be done effectively only with
  a web interface, which we won't have (probably for a long time); right
  now asking contributors to look up a category from a given list would
  be just a nuisance.
  Anyway, we could use a script to add this later based on the directory.
template: some rewording
template: remove two keywords

I think we should _only_ leave the most important keys in the template.

Additionally we should provide a list of accepted keys in the documentation
'we'd rather have undocumented snippets than no snippets'
merge branch snippet-template