Updates for Installation documentation

[sseide]

Hello - thanks for this helpful plugin but here are some remaks to improve the docs a bit:
I did a fresh installation for a test system and these steps where missing or not that clear from your current "get started" docs (http://redmine-git-hosting.io/get_started/)

Step 1) needed to install following packages too, otherwise later on the build of "rugged with native extension" will fail:

• pkg-config
• libssl-dev
  (system: debian 10 buster)

Here another sentence as a remark would probably be nice "Working with redmine_git_hosting plugin needs a running Gitolite3 installation. If you do not have one just check step 7 for basic installation instructions."
Its just a matter of order. Gitolite is a prerequisite, therefor should be mentioned at step 1 at least.

Step 2) Current versions does not need "redmine_bootstrap_kit" any more. Should get a remark along "only needed for redmine_git_hosting < 4.0" or similiar

Thanks,
Stefan Seide

[sseide]

as said inside #766:
add hint to disable "git" after all repositories have move to new "Xitolite"/"Gitolite" backend inside step 9 from "Get Started" installation documentation

[Vitexus]

Also upgrade documentation is a bit buggy:
http://redmine-git-hosting.io/how-to/upgrade/

redmine@vitexsoftware:~/plugins/redmine_git_hosting$ git fetch -p
From https://github.com/jbox-web/redmine_git_hosting
 - [deleted]           (none)     -> origin/devel
   (refs/remotes/origin/HEAD has become dangling)
 - [deleted]           (none)     -> origin/feat_new_migrations
 - [deleted]           (none)     -> origin/feat_regenerate_keys
remote: Enumerating objects: 1140, done.
remote: Counting objects: 100% (1140/1140), done.
remote: Compressing objects: 100% (267/267), done.
remote: Total 1771 (delta 898), reused 1063 (delta 837), pack-reused 631
Receiving objects: 100% (1771/1771), 262.89 KiB | 2.35 MiB/s, done.
Resolving deltas: 100% (1222/1222), completed with 313 local objects.
   ea6a1428..d273b22d  gh-pages   -> origin/gh-pages
   c27352b9..a219496f  master     -> origin/master
redmine@vitexsoftware:~/plugins/redmine_git_hosting$ git checkout 5.0.0
error: pathspec '5.0.0' did not match any file(s) known to git