Add .envrc

https://direnv.net/ with use nix, requires the shell.nix to be placed at
the project root. This is a very common setup.

Change-Id: I6eff8cca03aaa35c68a034ba1f553946c893377c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/317
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/.envrc b/.envrc
new file mode 100644
index 0000000..1d953f4
--- /dev/null
+++ b/.envrc
@@ -0,0 +1 @@
+use nix
diff --git a/util/shell.nix b/shell.nix
similarity index 100%
rename from util/shell.nix
rename to shell.nix