Update "Stash" documentation

According to the Stash documentation, users should be able to link to a Stash repository using the administration menu. This is currently not possible in Bamboo Cloud (OnDemand). The Atlassian Bamboo Stash Repository Plugin must be enabled so users can link to a Stash repository the way that the documentation describes, however it's not possible to have that plugin enabled because Bamboo OnDemand can't use SSH transport, and this plugin only supports SSH transport - so enabling it on OD is not really an option. There's a request here to have the plugin enabled.

Users should still be able to connect to a Stash repo from Bamboo Cloud, but not this way. Instead, users should go to the clone tab from their Stash repo, get the http or https clone address, go back to Bamboo and add a repo (e.g. type: Git) using that address. When adding a repo, under the Advanced options, there's a "Web repository" option, from there users can select Stash. It will checkout from stash the same as it would from any other repo, but won't have the cool stash integration features you would get if you have the plugin enabled.