scripts/bw-compatibility-test: add backwards compat test

In this commit, a new backwards compatibility test is added. See the
added README.md file in this commit for all the info.

scripts/bw-compatibility-test/README.md

@@ -0,0 +1,125 @@
+# Basic Backwards Compatibility Test
+
+This directory houses some docker compose files and various bash helpers all 
+used by the main `test.sh` file which runs the test. The idea is to be able to 
+test that a node can be upgraded from a stable release to a checked out branch
+of LND and still function as expected with nodes running the stable release.
+
+## Test Flow
+
+The test sets up the following network:
+
+```
+Alice <---> Bob <---> Charlie <---> Dave
+```
+
+Initially, all the nodes are running a tagged LND release. This is all 
+configured via the main `docker-compose.yaml` file. 
+
+1. The Bob node is the node we will focus on. While Bob is still on a stable 
+   version of LND, we ensure that he can: send and receive multi-hop payments as
+   well as route payments.
+
+2. Bob is then shutdown. 
+
+3. The `docker-compose.override.yaml` file is then loaded and used to spin up
+   Bob again but this time using the checked out branch of LND. This is done by 
+   using the `dev.Dockerfile` in the LND repo. 
+
+4. The test now waits for this new version of Bob (which uses the same data 
+   directory as the previous version) to sync up with the network, reactivate 
+   its channels.
+ 
+5. Finally, basic send, receive and routing tests are run to ensure that Bob
+   is still functional after the upgrade. 
+
+## How to use this directory
+
+1. If you would just like to run the full test from start to finish, then all 
+   you need to do is run: 
+  
+   ```bash
+    ./test.sh
+   ```
+   
+2. If you would like to run the test in parts, then you can use the `execute.sh`
+   script to call the various functions in the directory. Here is an example:
+   ```bash
+   # Spin up the docker containers.
+   ./execute.sh compose_up
+   
+   # Wait for the nodes to start.
+   ./execute.sh wait_for_nodes alice bob charlie dave
+   
+   # Query various nodes.
+   ./execute.sh alice getinfo
+   
+   # Set-up a basic channel network.
+   ./execute.sh setup-network
+   
+   # Wait for bob to see all the channels in the network.
+    ./execute.sh wait_graph_sync bob 3
+   
+   # Open a channel between Bob and Charlie.
+   ./execute.sh open_channel bob charlie
+   
+   # Send a payment from Alice to Dave.
+   ./execute.sh send_payment alice dave
+   
+   # Take down a single node. 
+   ./execute.sh compose_stop dave
+   
+   # Start a single node.
+   ./execute.sh compose_start dave
+   
+   ```
+
+## File Descriptions:
+
+##### `test.sh`
+
+This script runs the full backwards compatibility test. 
+
+ ```bash
+   ./test.sh
+ ```
+
+##### `execute.sh`
+
+A helper script that allows you to call the various functions in the directory.
+
+This is useful if you are debugging something and want to step through the test
+steps manually while keeping the main network running.
+
+For example: 
+```bash
+ # Spin up the docker containers.
+ ./execute.sh compose_up
+ 
+ # Query various nodes.
+ ./execute.sh alice getinfo
+```
+
+#### `compose.sh`
+
+This script contains all the docker-compose variables and helper functions that 
+are used in the test.
+
+#### `network.sh`
+
+This script contains all the various helper functions that can be used to 
+interact with the Lightning nodes in the network.
+
+### `vars.sh`
+
+Any global variables used across the other scripts are defined here.
+
+#### `docker-compose.yaml`
+
+This docker compose file contains the network configuration for all the nodes
+running a stable LND tag. 
+
+#### `docker-compose.override.yaml`
+
+This compose file adds defines the `bob-pr` service which will pull and build
+the LND branch that is currently checked out.