From 85626e6d89172764946a71b2fab3bd19e00bb3b6 Mon Sep 17 00:00:00 2001
From: Louis Chatriot <louis.chatriot@gmail.com>
Date: Tue, 28 May 2013 15:22:24 +0200
Subject: [PATCH] Update README.md

---
 README.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/README.md b/README.md
index 39b18ed..38f749c 100644
--- a/README.md
+++ b/README.md
@@ -189,6 +189,8 @@ db.find({ $or: [{ planet: 'Earth' }, { planet: 'Mars' }], inhabited: true }, fun
   * `numReplaced` is the number of documents replaced
   * `upsert` is set to true if the upsert mode was chosen and a document was inserted
 
+**Note**: you can't change a document's _id.
+
 ```javascript
 // Let's use the same example collection as in the "finding document" part
 // { _id: 'id1', planet: 'Mars', system: 'solar', inhabited: false }