Adding text about tree

docs/filesystem.md

@@ -6,7 +6,7 @@ This section is going to be a high-level talk about the Linux filesystem concept
 
 The Linux filesystem directory structure starts with the top root directory, which is shown as <code>/</code>. Below this are several other standard directories. Of particular interest are <code>usr/bin</code>, <code>home</code>, <code>usr/lib</code>, and <code>usr/lib64</code>. A common directory which you will also often find is <code>usr/local/bin</code>. 
 
-The picture on the right shows typical subdirectories under <code>/</code> (note that the command 'tree' does not work at all HPC centres). Some of the directories have a symbolic link to a different name - this is often done to make it quicker to write, but can also be for compatibility reasons since some software have hardcoded paths. 
+The picture on the right shows typical subdirectories under <code>/</code> (note that the command 'tree' does not work at all HPC centres - see the page [tree](#tree) under "Extras section for how to install if it is missing). Some of the directories have a symbolic link to a different name - this is often done to make it quicker to write, but can also be for compatibility reasons since some software have hardcoded paths. 
 
 - **usr/bin**: contains (most) of the system-specific binaries
 - **usr/local/bin**: non-system binaries. often locally compiled/maintained packages