readme: Use https for riscv clone

Clone example used git@github which requires permissions to clone repository, preventing anonymous cloning. github recommends https as it is smart and will use appropriate access for your permissions. This will help people to get started. See https://help.github.com/articles/which-remote-url-should-i-use/#cloning-with-https-recommended
Merge pull request #1 from docrjp/master

readme: Use https for riscv clone