Add DEVELOP.md for development instructions

Signed-off-by: Gavin Zhao <gavinz@brown.edu>

DEVELOP.md

@@ -0,0 +1,30 @@
+This document provides instructions on how to setup your environment to
+develop/hack on ATS-Xanadu.
+
+## Prerequisites
+
+- The [`nix`](https://nixos.org/download/) package manager.
+
+That's it! The hard job of setting up dependencies and configuring environment
+variables are offloaded to Nix
+([flakes](https://zero-to-nix.com/concepts/flakes/)), all while making sure
+these changes are isolated for this project only and will not affect your
+system.
+
+## Steps
+
+> [!WARNING]
+> For advanced Make users: do **NOT** add `-j` flag to build in parallel. There
+are still issues regarding the way compiler outputs are merged together.
+The build is quick and shouldn't take more than 10 minutes to complete
+even when building with one core.
+
+1. Run `nix develop`. You will be dropped into a bash shell that contains all
+   the dependencies and environment variables configured for you.
+
+   The first run may take a while, because Nix needs to fetch dependencies.
+   Subsequent `nix develop` runs are much faster due to caching.
+
+2. Run `make -f Makefile_overall`.
+
+3. Now you have a working ATS3 compiler! Checkout the `PGROUND` for examples.

PGROUND/proj002@250110/srcgen1/DATS/brainfxxk.dats

@@ -22,6 +22,8 @@ excptcon Underflow of ()
 excptcon RBNotFound of ()
 excptcon LBNotFound of ()
 
+
+
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