[Esip-discovery] added a new section on search-specific terms

[Lynnes, Christopher S. (GSFC-6102)]

7.2.9	Optional Nature of Search-specific Url Template Fields (Required)

Optional placeholders for search-specific attributes can be specified by adding ‘?’ to the end of the search term, e.g., http://example.com/search/{instrument?}.
However the search-specific attributes themselves are not considered themselves to be part of the specification.  Also, such attributes MUST be optional; required search-specific terms that are not spelled out in the ESIP OpenSearch specification render the associated search service non-compliant with that convention.
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Dr. Christopher Lynnes, NASA/GSFC, ph: 301-614-5185