Add riscv64 to outputs list in release-process.md

The riscv64 binary is created by the Gitian scripts and distributed by the
Bitcoin Core website, so it should be listed in the release process docs.

doc/release-process.md

@@ -270,6 +270,7 @@ The list of files should be:
 bitcoin-${VERSION}-aarch64-linux-gnu.tar.gz
 bitcoin-${VERSION}-arm-linux-gnueabihf.tar.gz
 bitcoin-${VERSION}-i686-pc-linux-gnu.tar.gz
+bitcoin-${VERSION}-riscv64-linux-gnu.tar.gz
 bitcoin-${VERSION}-x86_64-linux-gnu.tar.gz
 bitcoin-${VERSION}-osx64.tar.gz
 bitcoin-${VERSION}-osx.dmg