Update README.md

README.md

-# Istex
+# Istex API Crawler
+
 ## API documentation
 https://doc.istex.fr/api/
+
+## Usage
+### Entry function
+The basic entry point of this crawler is the function `ISTEX.` `getMetadataWith`.
+
+This function take a `Text` representing the query you want to run on the istex API 
+and a `Maybe Int` representing the maximum number of values you want to get.
+### Return Type
+The return type is pretty simple, it's **either** a `ClientError` or a `ISTEX.` `Client.Documents`.
+
+`Documents` represent a collection of `ISTEX.` `Client.Document` and the number of `Document` returned.
+
+`Document` is a simple type that contain every informations we need (id, title,authors,abstract,publicationDate,sources).
+### Exemple
+Here is a basic main using the entry point of the crawler and printing the 5 first results.
+```hs
+{-# LANGUAGE OverloadedStrings #-}
+
+
+import ISTEX
+import ISTEX.Client
+
+main :: IO ()
+main = do
+  res <- getMetadataWith "artificial intelligence" (Just 10)
+  case res of
+    (Left err) -> print "Error"
+    (Right val) -> do
+      print $ take 5 val
+```
+