Add note about checking out LFS assets

README.md

@@ -8,6 +8,9 @@ Note: Until Godot 3.1-stable is released, you need a recent build from Godot's
 *master* branch to run this demo. You can build the engine from source, or use one
 of @Calinou's [nightly builds](https://hugo.pro/projects/godot-builds).
 
+Note 2: The repository is big and asset importing not well optimized yet, so expect
+a high CPU and RAM load when opening the project for the first time.
+
 ### Git LFS
 
 This demo uses [Git LFS](https://git-lfs.github.com/) to store the heaviest assets.
@@ -16,6 +19,7 @@ You need to install Git LFS on your system, and then fetch the assets from the L
 repository with:
 ```
 git lfs fetch
+git checkout master
 ```
 
 ### License