Merge pull request freqtrade#11747 from nisuhw/patch-1

Update installation.md for older Raspberry Pi 2

Author: xmatthias

docs/installation.md

@@ -87,7 +87,7 @@ OS Specific steps are listed first, the common section below is necessary for al
 
 
     ```bash
-    sudo apt-get install python3-venv libatlas-base-dev cmake curl
+    sudo apt-get install python3-venv libatlas-base-dev cmake curl libffi-dev
     # Use piwheels.org to speed up installation
     sudo echo "[global]\nextra-index-url=https://www.piwheels.org/simple" > tee /etc/pip.conf