Add information in the README about the test LEAK_CHECK functionality

Author: Nao-ris

README.md

@@ -82,6 +82,13 @@ $ cargo test --features=enterprise
 
 ### Run with Couchbase Lite C leak check
 
+Couchbase Lite C allows checking if instances of their objects are still alive through the functions `CBL_InstanceCount` & `CBL_DumpInstances`.
+If the `LEAK_CHECK` environment variable is set, we check that the number of instances at the end of each test is 0.
+
+If this step fails in one of your pull requests, you should look into the `wrap`/`retain` logic on CBL pointers in the constructor of the Rust structs:
+- `wrap` takes ownership of the pointer, it will not increase the ref count of the `ref` CBL pointer so releasing it (in a `drop`) will destroy the pointer
+- `retain` just references the pointer, it will increase the ref count of CBL pointers so releasing it will not destroy the pointer
+
 ```shell
 $ LEAK_CHECK=y cargo test --features=enterprise -- --test-threads 1
 ```