Add reverse edge query test and update README

[matthewmcneely]

Description

This PR adds a reverse edge query test and updates the README

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to
  this PR

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[Copilot]

Pull Request Overview

This PR introduces a new integration test for Dgraph’s reverse-edge querying and expands the client interface with additional CRUD and schema methods.

• Defines Student and Class types with reverse-edge tags and adds TestReverseEdgeQuery to verify reverse-edge behavior.
• Extends the Client interface in client.go, adding methods like Insert, Upsert, Get, Query, UpdateSchema, and DgraphClient.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
query_test.go	Added Student/Class structs and TestReverseEdgeQuery to cover reverse-edge query functionality.
client.go	Updated Client interface by adding new CRUD, schema, and utility methods.

Comments suppressed due to low confidence (2)

query_test.go:335

• [nitpick] Go naming conventions discourage underscores in exported field names; consider renaming Takes_Class to TakesClass for clarity and consistency.

Takes_Class []*Class `json:"takes_class,omitempty" dgraph:"reverse"`

client.go:42

• [nitpick] The Query method returns a query builder rather than executing immediately; consider renaming to NewQueryBuilder or adding an Execute method on the client for clarity.

Query(context.Context, any) *dg.Query