Add an example usage of the library in the README

README.md

@@ -25,7 +25,68 @@ This repository contains crates to quickly debug the engine:
 
 ## How to use it?
 
-_Section in WIP_
+Milli is a library that does search things, it must be embedded in a program.
+You can compute the documentation of it by using `cargo doc --open`.
+
+Here is an example usage of the library where we insert documents into the engine
+and search for one of them just after.
+
+```rust
+let path = tempfile::tempdir().unwrap();
+let mut options = EnvOpenOptions::new();
+options.map_size(10 * 1024 * 1024); // 10 MB
+let index = Index::new(options, &path).unwrap();
+
+let mut wtxn = index.write_txn().unwrap();
+let content = documents!([
+    {
+        "id": 2,
+        "title": "Prideand Prejudice",
+        "au{hor": "Jane Austin",
+        "genre": "romance",
+        "price$": "3.5$",
+    },
+    {
+        "id": 456,
+        "title": "Le Petit Prince",
+        "au{hor": "Antoine de Saint-Exupéry",
+        "genre": "adventure",
+        "price$": "10.0$",
+    },
+    {
+        "id": 1,
+        "title": "Wonderland",
+        "au{hor": "Lewis Carroll",
+        "genre": "fantasy",
+        "price$": "25.99$",
+    },
+    {
+        "id": 4,
+        "title": "Harry Potter ing fantasy\0lood Prince",
+        "au{hor": "J. K. Rowling",
+        "genre": "fantasy\0",
+    },
+]);
+
+let config = IndexerConfig::default();
+let indexing_config = IndexDocumentsConfig::default();
+let mut builder =
+    IndexDocuments::new(&mut wtxn, &index, &config, indexing_config.clone(), |_| ())
+        .unwrap();
+builder.add_documents(content).unwrap();
+builder.execute().unwrap();
+wtxn.commit().unwrap();
+
+
+// You can search in the index now!
+let mut rtxn = index.read_txn().unwrap();
+let mut search = Search::new(&rtxn, &index);
+search.query("horry");
+search.limit(10);
+
+let result = search.execute().unwrap();
+assert_eq!(result.documents_ids.len(), 1);
+```
 
 ## Contributing