Add doc for atomic update

Documentation/Atomic-updates.md

@@ -0,0 +1,46 @@
+# Atomic updates
+Atomic updates can be used update only specific fields in document. This can help speed up indexing in certain use cases. With SolrNet, atomic updates can be done using `AtomicUpdate()` methods.
+
+Each individual update is defined by the `AtomicUpdateSpec`, which has below fields
+- `field` - Name of field
+- `type` - Type of operation to be done
+- `value` - Value to be updated
+
+`AtomicUpdate()` method requests update on a single document, which can be identified by the first parameter to the method.
+
+### Example
+
+Consider a sample document of type `Product` as below,
+
+```C#
+public class Product {
+    [SolrUniqueKey("id")]
+    public string Id { get; set; }
+
+    [SolrField("cat")]
+    public ICollection<string> Categories { get; set; }
+
+    [SolrField("sold")]
+    public int SoldCount { get; set; }
+
+    [SolrField("inStock")]
+    public int InStock { get; set; }
+}
+```
+
+Below code can be used to update a document with id `mydoc` by adding a new category `toys` and to increment the `SoldCount` by 14 and decrement `InStock` count by 6.
+
+```
+ISolrOperations<Product> solr = ...
+solr.AtomicUpdate("mydoc",
+  new[]
+  {
+      new AtomicUpdateSpec("cat", AtomicUpdateType.Add, "toys"),
+      new AtomicUpdateSpec("sold", AtomicUpdateType.Inc, 14),
+      new AtomicUpdateSpec("inStock", AtomicUpdateType.Inc, -6),
+  });
+```
+
+### Reference
+
+See the [Solr wiki](https://solr.apache.org/guide/solr/latest/indexing-guide/partial-document-updates.html#atomic-updates) for more information.

README.md

@@ -40,6 +40,7 @@ The easiest way to get going is to use our NuGet packages:
  * [Mapping](Documentation/Mapping.md)
  * [Initialization](Documentation/Initialization.md)
  * [Create/Update/Delete](Documentation/CRUD.md)
+ * [Atomic updates](Documentation/Atomic-updates.md)
  * [Querying](Documentation/Querying.md)
  * [Faceting](Documentation/Facets.md)
  * [Highlighting](Documentation/Highlighting.md)