JSDoc review

Just another file by file consistency review, making sure all blocks are
formatted the same way, that @ tags all follow the description, no
superfluous private blocks, valid @type and @default declarations,
indentation, sentences on separate lines wrapped at 80 columns, no
trailing double-spaces after sentences.
Add contributing file.
Add a Core header to separate the sections.
Merge pull request #1424 from francoisfrisch/contributing

Add contributing file.
Merge pull request #1370 from kriskowal/jsdoc-review

JSDoc review
Merge pull request #1425 from aadsm/fix-repetition-selection

Allow repetition to be selected with no components
Tag components with data-montage-le-component
Add _addTemplateObjects
Tag named and star argument nodes

Named arguments have a single attribute: data-montage-le-arg.
Star arguments are a range, the first element has the
data-montage-le-being attribute and the last element data-montage-le-end.
Add templateDocumentPart to iteration
Add parentDocumentPart to DocumentPart