add a quick switch page for current developers

src/SUMMARY.md

@@ -30,6 +30,7 @@
 
 # Developing
 - [Getting Started](developing/getting-started.md)
+  - [Quick Switch to denv and just](developing/quick-switch.md)
 - [Building and Installing ldmx-sw](developing/building/intro.md)
   - [Shared Computing Clusting](developing/building/clusters.md)
   - [Updating ldmx-sw](developing/building/updating.md)

src/developing/quick-switch.md

@@ -0,0 +1,25 @@
+# Quick Switch to `denv`+`just`
+
+No frills, no explanations.
+
+If anything goes wrong, fall back to the more full getting
+started guides for [using](../using/getting-started.md) (for details on `denv`)
+or [developing](getting-started.md) (for details on `just`).
+
+This is targeted towards LDMX contributors who were present
+before the transition to `denv`+`just` so they already have
+a container runner present on their machine.
+
+```sh
+# install denv into ~/.local
+curl -s https://raw.githubusercontent.com/tomeichlersmith/denv/main/install | sh 
+# install just into the same location
+curl --proto '=https' --tlsv1.2 -sSf https://just.systems/install.sh |\
+  bash -s -- --to ~/.local/bin
+# add just tab completion to your ~/.bashrc
+printf '\neval "$(just --completions bash)"\n' >> ~/.bashrc
+```
+
+The `denv` install script attempts to help you update your `~/.bashrc`
+so that `~/.local/bin` is in your `PATH` and discoverable by `bash` as
+a program, but you may need to update your `PATH` manually.