README: document that env vars work for any deployment

The three BASE_URL variables are not limited to CD and production — they
work for any live deployment.  Add an explicit note and a concrete example
using a personal/feature instance so this is clear to new contributors.

1 files changed, 13 insertions, 0 deletions

diff --git a/README.md b/README.md
index 69ba217..e932a3d 100644
--- a/README.md
+++ b/README.md
@@ -33,6 +33,19 @@ GN_AUTH_BASE_URL=https://auth.genenetwork.org \
 pytest -m "smoke"
 ```
 
+The three variables work for **any deployment** — CD, production, or a
+personal/feature instance.  To run against an arbitrary stack, just point
+the variables at it:
+
+```sh
+GN2_BASE_URL=https://gn2-fred.genenetwork.org \
+GN3_BASE_URL=https://gn2-fred.genenetwork.org/api3 \
+GN_AUTH_BASE_URL=https://gn2-fred-auth.genenetwork.org \
+pytest -m "smoke"
+```
+
+No test-code changes are needed — all URL configuration lives in `conftest.py`.
+
 ## Test marks
 
 | Mark | Description |