Add note about sparse files
diff --git a/README.md b/README.md
index 73c4a43..c20f470 100644
--- a/README.md
+++ b/README.md
@@ -62,3 +62,5 @@
This runs as root and auto-`sudo`s itself to achieve that. As such, you shouldn't run this on paths you don't trust or paths that could be controlled by malicious users.
Since `ctmg` uses `cryptsetup` and the LUKS infrastructure, it uses the Linux block device encryption APIs. The state of the art in block device encryption, as of writing, is [XTS mode](http://csrc.nist.gov/publications/nistpubs/800-38E/nist-sp-800-38E.pdf), which is what `ctmg` uses. But do note that this does not guarantee, entirely, the integrity of data, just the secrecy. As such, if a malicious user is able to modify the encrypted content, it is possible this could result in differing decrypted content without you noticing. So, `ctmg` is useful for keeping things secret, but not for guaranteeing the authenticity of the data. If your laptop gets stolen, sleep safely knowing that your `ctmg`-secured data is safe, but if an attacker is actively modifying the `.ct` file while you're using it in one way or another, you've got trouble.
+
+In order to conserve space, `ctmg` uses `truncate` to make sparse files. This means that the file grows as it's used. An attacker can therefore see how much of the container is utilized. If you care about this, it's easy enough to replace the single call to `truncate` with a single call to `dd if=/dev/urandom` to make a completely full file containing only random data.
