README.md: Add tips for make check

Problem: `make check` in flux-core takes a very long time to complete
without use of `-j`, but the README doesn't mention or suggest use
of this option.

Add `-jN` to the options of both `make` and `make check` in the
instructions. Also add a tip suggesting the use of `-j` with `make
check` specifically, and add a link to the testsuite README.

Author: grondo

README.md

@@ -33,10 +33,14 @@ are available for [redhat](scripts/install-deps-rpm.sh) and
 ```
 ./autogen.sh   # skip if building from a release tarball
 ./configure
-make
-make check
+make -jN
+make -jN check
 ```
 
+> [!TIP]
+> `make check` runs many tests. Use of `-jN` recommended.
+> For more details about the testsuite, see the [README](t/README.md).
+
 ##### VSCode Dev Containers
 
 If you use VSCode we have a dev container and [instructions](vscode.md).