Add "make clean" to build instructions

When I tried to build a new version in the same working directory I'd used to build a previous version, I saw build errors. After a few attempts, I remembered that "make clean" might get past the problem. It seems like a nice thing to suggest to everyone who might fall into the same trap.

README.md

@@ -43,6 +43,7 @@ docker run --rm -i -v "$PWD:$PWD" -w "$PWD" ghcr.io/jqlang/jq:latest '.version'
 git submodule update --init # if building from git to get oniguruma
 autoreconf -i               # if building from git
 ./configure --with-oniguruma=builtin
+make clean        # if upgrading from a version previously built from source
 make -j8
 make check
 sudo make install