Subversive update instructions

The user is able to update Subversive using Eclipse update manager and the same steps as for installation procedure. The only difference is skipping steps 3 and 4.

Also note that the user is able to update versions in the same release stream, i.e. 1.1.x or 1.0.x. For example you are able to update 1.1.1 to 1.1.3, but can't update 1.0.0 to 1.1.3. If the user wants to change release stream, he has to to register a new update site, which corresponds to new release stream and install Subversive using it.