Adding conditional upsert docs (#76)

* Adding conditional upsert docs

Author: prashant-shahi

README.md

@@ -27,7 +27,8 @@ and understand how to run and work with Dgraph.
   - [Creating a transaction](#creating-a-transaction)
   - [Running a mutation](#running-a-mutation)
   - [Running a query](#running-a-query)
-  - [Running an Upsert: Query + Mutation](#running-an-upsert-query--mutation)
+  - [Running an upsert: query + mutation](#running-an-upsert-query--mutation)
+  - [Running a conditional upsert](#running-a-conditional-upsert)
   - [Committing a transaction](#committing-a-transaction)
   - [Cleanup Resources](#cleanup-resources)
   - [Debug mode](#debug-mode)
@@ -255,7 +256,7 @@ const res = await txn.doRequest(req);
 console.log(JSON.stringify(res.getJson()));
 ```
 
-### Running an Upsert: Query + Mutation
+### Running an upsert: query + mutation
 
 The `txn.doRequest` function allows you to run upserts consisting of one query and one mutation. 
 Query variables could be defined and can then be used in the mutation. You can also use the 
@@ -277,7 +278,33 @@ req.setQuery(query);
 req.setMutationsList([mu]);
 req.setCommitNow(true);
 
-// Update email only if matching uid found.
+// Upsert: If wrong_email found, update the existing data
+// or else perform a new mutation.
+await dgraphClient.newTxn().doRequest(req);
+```
+
+### Running a conditional upsert
+
+The upsert block allows specifying a conditional mutation block using an `@if` directive. The mutation is executed
+only when the specified condition is true. If the condition is false, the mutation is silently ignored.
+
+See more about Conditional Upsert [Here](https://docs.dgraph.io/mutations/#conditional-upsert).
+
+```js
+const query = `
+  query {
+      user as var(func: eq(email, "[email protected]"))
+  }`
+
+const mu = new dgraph.Mutation();
+mu.setSetNquads(`uid(user) <email> "[email protected]" .`);
+mu.setCond(`@if(eq(len(user), 1))`);
+
+const req = new dgraph.Request();
+req.setQuery(query);
+req.addMutations(mu);
+req.setCommitNow(true);
+
 await dgraphClient.newTxn().doRequest(req);
 ```