Add information on index joins to documentation
Add information on index joins to documentation

Motivation
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The new index join feature for Couchbase Server 4.5 was added in #133.
However, there is no documentation.

Modifications
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Added feature documentation in joins.md.

Results
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This type of join is now documented.

drGarbinsky/Linq2Couchbase,brantburnett/Linq2Couchbase,couchbaselabs/Linq2Couchbase