Update installation guide for 2.1. Based on orchestral/platform#29
issue.

Signed-off-by: crynobone <crynobone@gmail.com>
Guide should suggest stable version when possible.

Signed-off-by: crynobone <crynobone@gmail.com>
Merge branch '2.0' into 2.1

Signed-off-by: crynobone <crynobone@gmail.com>

Conflicts:
	src/installation.md