Added bhvrSession.md

Doc/bhvrSession.md

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+## bhvrSession Cookie
+
+The Steam Login Function (see Steam_Login.md) returns the Cookie bhvrSession with the Value UserID (User ID is a custome Value generated by the MongoDB handler).
+
+The Client then adds this Cookie to every following request.
+
+This is done so that the API knows which user data to return.
+
+# Login Endpoint
+/api/v1/auth/provider/steam/login
+
+# Retrieving Cookie
+
+```python
+userid = request.cookies.get('bhvrSession')
+```
+# Usage example
+Request
+```python
+@app.route("/api/v1/inventories", methods=["GET"])
+def inventories():
+    userid = request.cookies.get('bhvrSession')
+    return jsonify({"code": 200, "message": "OK", "data": {"playerId": userid, "inventory": []}})
+```