Adding OpenSearch description for searching

documentation/usage-guide/searching.md

 # Searching
+
+The main searching interface is supported using OpenSearch using
+various extension.
+
+The OpenSearch URL endpoints are:
+
+ * `/opensearch`: Main service description
+ * `/opensearch/<format>`: Search for collections in a specific format
+ * `/opensearch/collections/<collection-id>`: Collection specific description
+ * `/opensearch/collections/<collection-id>/<format>`: Search for products in a collection in that format
+
+
+The description related entrypoints provide an OpenSearch Description
+Document (OSDD). This XML based file format describes general service metadata and various access URLs.
+
+These URL objects provide templates that can be filled with
+parameters to submit actual metadata queries. Depending on the used
+URL, the search result is returned in one of the following formats:
+
+ * `atom`: Atom syndication format
+ * `rss`: RSS
+ * `kml`: Keyhole Markup Language
+ * `json`: GeoJSON
+
+The resulting items in that result document can be paged. Specific
+pages can be retrieved using the `offset` and `count` parameters.
+Additionally, the response contains links to the first, last, next,
+and previous pages of the result set.
+
+The results of the search depend on the type of object being searched.
+For the collection search, the result items are collection descriptions
+of all matched collections encoded in the selected format. Each
+description contains a link to its own specific OSDD document.
+
+In product searches, the result document items are descriptions of
+products, containing links to thumbnails, browses and product specific
+services such as WMS or WCS. Additionally it contains the products
+acquisition footprint and timestamp.
+
+## Filters
+
+The URL templates for both the collection and the record search allow
+to pass parameters that control the query and shape of the search.
+The following table shows all parameters available for both collection
+and product search:
+
+| Parameter     | Description                                                                                   |
+| ------------- | --------------------------------------------------------------------------------------------- |
+| startIndex    | The index offset of the items. Used to get subsets of a result set                            |
+| count         | The number of records for each page                                                           |
+| start         | The start timestamp for the time range                                                        |
+| end           | The end timestamp for the time range                                                          |
+| timerel       | The semantic of the time range. Either `intersects`, `contains`, `disjoint`, or `equals`      |
+| bbox          | The geographic bounding box expressed as `min-lon`, `min-lat`, `max-lon`, `max-lat` in WGS 84 |
+| geom          | A WKT encoded geometry for a geographic search                                                |
+| lon           | The longitude component of point/radius searches                                              |
+| lat           | The latitude component of point/radius searches                                               |
+| r             | The radious in metres of point/radius searches                                                |
+| georel        | The semantic of the geometry filter. One of `intersects`, `contains`, or `disjoint`           |
+
+The following parameters are only available in collection searchesin
+addition the general search parameters:
+
+| Parameter     | Description                                                                                   |
+| ------------- | --------------------------------------------------------------------------------------------- |
+
+
+The following parameters are only available in product searches in
+addition the general search parameters:
+
+| Parameter     | Description                                                                                   |
+| ------------- | --------------------------------------------------------------------------------------------- |
+| cloudCover    | Cloud cover filter. Either a specific value or an interval of values. Values between 0-100    |
+