Create Extract.md

Documentation/Extract.md

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+# Binary document upload
+
+SolrNet supports Solr "extract" feature (a.k.a. Solr "Cell") to index data from binary document formats such as Word, PDF, etc.
+
+Here's a simple example showing how to extract text from a PDF file, without indexing it:
+
+```csharp
+ISolrOperations<Something> solr = ...
+using (var file = File.OpenRead(@"test.pdf")) {
+    var response = solr.Extract(new ExtractParameters(file, "some_document_id") {
+        ExtractOnly = true,
+        ExtractFormat = ExtractFormat.Text,
+    });
+    Console.WriteLine(response.Content);
+}
+```
+
+`ExtractOnly = true` tells Solr to just perform text extraction but not index the uploaded document.
+If `ExtractOnly = false` you can add more fields with the `Fields` property. 
+Other options can be set through the properties of the [`ExtractParameters` class](https://github.com/mausch/SolrNet/blob/master/SolrNet/ExtractParameters.cs).
+It's usually recommended to provide the `StreamType` for the content, as auto-detection might fail.
+
+For more details about each option in `ExtractParameters` see the [Solr wiki](https://wiki.apache.org/solr/ExtractingRequestHandler) and the [Solr reference guide](https://cwiki.apache.org/confluence/display/solr/Uploading+Data+with+Solr+Cell+using+Apache+Tika).