[docs] explain how to recompile only what bitcoind tests need

`make` rebuilds the entire project. This is quite slow if e.g. you're making changes to one file and only wish to run the bitcoind tests.

This commit adds an instruction to run `make -C src/test` (as opposed to `make src/test` and `make src/test/test_bitcoin`).

Author: Sjors

src/test/README.md

@@ -5,7 +5,10 @@ and tests weren't explicitly disabled.
 
 After configuring, they can be run with `make check`.
 
-To run the bitcoind tests manually, launch `src/test/test_bitcoin`.
+To run the bitcoind tests manually, launch `src/test/test_bitcoin`. To recompile
+after a test file was modified, run `make` and then run the test again. If you
+modify a non-test file, use `make -C src/test` to recompile only what's needed
+to run the bitcoind tests.
 
 To add more bitcoind tests, add `BOOST_AUTO_TEST_CASE` functions to the existing
 .cpp files in the `test/` directory or add new .cpp files that