Add build system changes to release notes

doc/release-notes.md

@@ -4,3 +4,17 @@ release-notes at release time)
 Notable changes
 ===============
 
+Build system
+------------
+
+- The `--enable-lcov`, `--disable-tests`, and `--disable-mining` flags for
+  `zcutil/build.sh` have been removed. You can pass these flags instead by using
+  the `CONFIGURE_FLAGS` environment variable. For example, to enable coverage
+  instrumentation (thus enabling "make cov" to work), call:
+
+  ```
+  CONFIGURE_FLAGS="--enable-lcov --disable-hardening" ./zcutil/build.sh
+  ```
+
+- The build system no longer defaults to verbose output. You can re-enable
+  verbose output with `./zcutil/build.sh V=1`