Add instructions on how to compile udunits from source (#329)

📝 Add instructions on how to compile udunits from source

Author: jemrobinson

docs/src/user-guide/installation.md

@@ -20,6 +20,25 @@ You will need [`uv`](https://docs.astral.sh/uv/getting-started/installation/) to
     export UDUNITS2_LIBDIR=/projects/u6iz/public/shared/udunits/lib/
     ```
 
+    Alternatively, if you prefer to compile `udunits` yourself, you can do the following:
+
+    ```bash
+    mkdir -p $HOME/software/src
+    cd $HOME/software/src
+
+    curl -L -o udunits-2.2.28.tar.gz https://downloads.unidata.ucar.edu/udunits/2.2.28/udunits-2.2.28.tar.gz
+    tar -xzf udunits-2.2.28.tar.gz
+    cd udunits-2.2.28
+
+    ./configure --prefix=$HOME/software/udunits-2.2.28
+    make -j4
+    make install
+
+    export UDUNITS2_XML_PATH=$HOME/software/udunits-2.2.28/share/udunits/udunits2.xml
+    export UDUNITS2_INCDIR=$HOME/software/udunits-2.2.28/include
+    export UDUNITS2_LIBDIR=$HOME/software/udunits-2.2.28/lib
+    ```
+
 Clone the repository and install with `uv`:
 
 ```bash