Add use case document

docs/usecase/usage-example.md

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+# Use cases
+
+  This document will introduce you to the example of what Vald can do.
+  Vald is a highly scalable distributed fast approximate nearest neighbor dense vector search engine, which uses [NGT](https://github.com/yahoojapan/NGT) as the core engine of Vald, and Vald manages to integrate with Kubernetes.
+
+  In general, you cannot search your unstructured data using the inverted index, like images and videos.
+  By applying the model like BERT or VGG, you can convert your unstructured data into vectors.
+  After converting them into vectors, you can insert it to Vald and process them in Vald.
+
+  Here are some general use cases of Vald or vector search engine.
+
+## Image and video processing
+
+  ![Image and video processing](../../assets/docs/usecase_image.png)
+
+  You can use Vald as the image/video processing engine to search the similar image/video or analyze the image/video for your use case.
+
+  Vald is capable to process a huge number of images at the same time so it case fit with your use case.
+
+  Here are some examples of what you can do with images and videos using Vald.
+
+- Search by image
+- Face recognition
+- Product recommendation based on images
+- Image/Video analysis
+- Image/Video deduplication
+
+## Audio processing
+
+  ![Audio processing](../../assets/docs/usecase_audio.png)
+
+  Audio processing is important for personal assistant implementation.
+
+  Vald can act as a brain of the personal assistant function, conversation interpreter and the natural language generation.
+
+  Here are some examples of what you can process using Vald.
+
+- Personal assistant
+- Speech recognition
+- Natural language understanding and generation
+
+## Text processing
+
+  ![Text processing](../../assets/docs/usecase_text.png)
+
+  Using a text vectorizing model like BERT, you can process your text data in Vald.
+
+  Here are some examples of the use case of text processing using Vald.
+
+- Search by text
+- Product recommendation based on text
+- Grammar checker
+- Real-time translator
+
+## Data analysis
+
+  ![Data analysis](../../assets/docs/usecase_data.png)
+
+  Vald can process the vector data, you can analyze every data you can vectorize.
+
+  Here are some examples of the use case of data analysis.
+
+- AI malware detection
+
+  To detect the malware using Vald, you need to vectorize the malware binary file and insert into Vald first.
+  You can analyze your binary by performing a search to find a similar binary in Vald.
+  If your binary is similar to the malware binary, you can trigger the alert for users.
+
+- Price optimization
+
+   By applying the price optimization technique using Vald, you can find the most optimized price for your business.
+  You can apply models like GLMs to achieve it and use Vald as a machine learning engine for your business.
+
+- Social analysis
+
+  To analyze the social relationship of users, you can suggest them their related friends, page recommendation, or other use cases.
+  You can apply different models to analyze the social data, and use Vald as a recommendation engine for your business.
+
+## Advanced use cases
+
+  Besides the general use case of Vald or vector search engine, Vald supports a user-defined filter that the user can customize the filter to filter the specific result.
+
+  For example when the user chose a man's t-shirt and the recommended product is going to be searched in Vald.
+
+  Without the filtering functionality, the women's t-shirt may be searched in Vald and displayed because women's t-shirt is similar to the men's t-shirt and it is very hard to differentiate the image of men's and women's t-shirt.
+
+  By implementing the custom filter, you can filter only the man's t-shirt based on your criteria and needs.