Explain ATHENA_PREFIX

src/docs/getting-started/quickstart.mdx

@@ -19,15 +19,15 @@ most recent version as of January 2021.
 *For an optimal experience, we strongly recommend you to use singularity 3 or later for this tutorial. 
 Use (very) old versions of singularity (e.g. 2.6.x) at your own risk!*
  
-## The EIC Software Container (jug_xl)
+## Step 1: Setup the EIC Software Container (jug_xl)
 
-### Step 1: Create a local directory that you want to work in, e.g., `~/eic`
+### Step 1a: Create a local directory that you want to work in, e.g., `~/eic`
 ```bash
 mkdir ~/eic
 cd ~/eic
 ```
 
-### Step 2: Deploy the containerized environment and launch a shell
+### Step 1b: Deploy the containerized environment and launch a shell
 
 Execute the following line in your terminal to setup your environment in your sandbox
 directory. This will install the latest stable container and place a script called
@@ -42,7 +42,14 @@ Now you can launch a shell in our development environment:
 ./eic-shell
 ```
 
-### Step 3: Clone the repos
+Note that starting `eic-shell` will set the environment variable `ATHENA_PREFIX` to point
+to the `local` subdirectory of your chosen working directory (e.g.,`~/eic/local`.
+This will then automatically add `$ATHENA_PREFIX/lib` to your `LD_LIBRARY_PATH` in the
+container. This means that anything you install to the `ATHENA_PREFIX` will be
+automatically available in your environment. This is important for DD4hep to find your
+custom installed detector plugins.
+
+## Step 2: Clone the repos
 
 There are 2 repos you need:
 * IP6 (IP-specific components)
@@ -57,7 +64,7 @@ ln -s ../ip6/ip6 athena/ip6
 In part 1 of the tutorial we will be mostly using files from a tutorial repo. 
 Later on, we will com back to these detector repositories.
 
-### Step 4: Looking at the geometry
+## Step 3: Looking at the geometry
 
 Before moving on to part 1, let's generate the geometry in a root file:
 

src/docs/part1/simple_detector.md

@@ -48,7 +48,7 @@ directories named compact (but not always).
 ### Compile the detector library
 
 ```bash
-cmake -B build -S . -DCMAKE_INSTALL_PREFIX=../local/ -DCMAKE_CXX_STANDARD=17
+cmake -B build -S . -DCMAKE_INSTALL_PREFIX=$ATHENA_PREFIX -DCMAKE_CXX_STANDARD=17
 cmake --build build -j4 -- install
 ```