docs: to README.md add tests troubleshooting (run in series)

Author: riverKanies

README.md

@@ -80,6 +80,20 @@ cargo update -p indexmap --precise "2.5.0"
 cargo update -p security-framework-sys --precise "2.11.1"
 ```
 
+## Troubleshooting Tests
+
+If tests are failing with `cargo test` it could be an issue related to tests that use bitcoind or electrsd running in parallel. To force tests to run in series you can try:
+
+```
+cargo test -- --test-threads=1
+```
+
+or
+
+```
+RUST_TEST_THREADS=1 cargo test
+```
+
 ## License
 
 Licensed under either of