Add documentation on how to implement custom index provider plugins

[lassewesth]

@BGehrels: 'There does not seem to be documentation on that issue.'

[lassewesth]

---

strings:

• "This issue was migrated from "
• ""
  values:
• https://github.com/neo4j/community/issues/876
  valueCount: 1

[jakewins]

This is definitely an area we'd like to improve, but for now, the documentation will remain the Interface you need to implement for indexes to work:
http://api.neo4j.org/1.8/org/neo4j/graphdb/index/IndexImplementation.html

We're looking over the overall indexing solution for the upcoming 2.0 release of Neo4j, and will take this into consideration as we do that work.