Update README.md

add tips to solve two errors:

$git push -u origin master
Access denied.
fatal: Could not read from remote repository.

and:

$ git push -u origin master
/usr/local/lib/ruby/2.0.0/net/http.rb:918:in `connect': SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed (OpenSSL::SSL::SSLError)
	from /usr/local/lib/ruby/2.0.0/net/http.rb:918:in `block in connect'
	from /usr/local/lib/ruby/2.0.0/timeout.rb:52:in `timeout'
	from /usr/local/lib/ruby/2.0.0/net/http.rb:918:in `connect'
	from /usr/local/lib/ruby/2.0.0/net/http.rb:862:in `do_start'
	from /usr/local/lib/ruby/2.0.0/net/http.rb:851:in `start'
	from /home/git/gitlab-shell/lib/gitlab_net.rb:62:in `get'
	from /home/git/gitlab-shell/lib/gitlab_net.rb:17:in `allowed?'
	from /home/git/gitlab-shell/lib/gitlab_shell.rb:60:in `validate_access'
	from /home/git/gitlab-shell/lib/gitlab_shell.rb:24:in `exec'
	from /home/git/gitlab-shell/bin/gitlab-shell:16:in `<main>'
fatal: Could not read from remote repository.