Add readme describing use of existing network

Author: maru-ava

tests/README.md

@@ -0,0 +1,28 @@
+# Developing with tmpnet
+
+The `load/` and `warp/` paths contain end-to-end (e2e) tests that use
+the [tmpnet
+fixture](https://github.com/ava-labs/avalanchego/blob/dev/tests/fixture/tmpnet/README.md). By
+default both test suites use the tmpnet fixture to create a temporary
+network that exists for only the duration of their execution.
+
+It is possible to create a temporary network that can be reused across
+test runs to minimize the setup cost involved:
+
+```bash
+# From the root of a clone of avalanchego, build the tmpnetctl cli
+$ ./scripts/build_tmpnetctl.sh
+
+# Start a new temporary network configured with subnet-evm's default plugin path
+$ ./build/tmpnetctl start-network \
+  --avalanche-path=./build/avalanchego
+  --plugin-dir=$GOPATH/src/github.com/ava-labs/avalanchego/build/plugins
+
+# From the root of a clone of subnet-evm, execute the warp test suite against the existing network
+$ ginkgo -vv ./tests/warp -- --use-existing-network --network-dir=$HOME/.tmpnet/networks/latest
+```
+
+The network started by `tmpnetctl` won't come with subnets configured,
+so the test suite will add them to the network the first time it
+runs. Subsequent test runs will be able to reuse those subnets without
+having to set them up.