Implement clarification about detached HEAD state

This commit implements changes discussed with @ben on
pull request #786 to make the text more easily understandable
for everyone.

Author: Thomas Hartmann

book/02-git-basics/sections/tagging.asc

@@ -211,7 +211,7 @@ Now, when someone else clones or pulls from your repository, they will get all y
 
 ==== Checking out Tags
 
-You can check out a tag through the `git checkout` command the same way you would with a commit:
+If you want to view the versions of files a tag is pointing to, you can do a git checkout, though this puts your repository in ``detached HEAD'' state, which has some ill side effects:
 
 [source,console]
 ----
@@ -232,4 +232,14 @@ HEAD is now at 99ada87... Merge pull request #89 from schacon/appendix-final
 $ git checkout 2.0-beta-0.1
 Previous HEAD position was 99ada87... Merge pull request #89 from schacon/appendix-final
 HEAD is now at df3f601... add atlas.json and cover image
-----
+----
+
+In ``detached HEAD'' state, if you make changes and then create a commit, the tag will stay the same, but your new commit won't belong to any branch and will be unreachable, except for by the exact commit hash. Thus, if you need to make changes—say you're fixing a bug on an older version, for instance—you will generally want to create a branch:
+
+[source,console]
+----
+$ git checkout -b version2 v2.0.0
+Switched to a new branch 'version2'
+----
+
+If you do this and make a commit, your `version2` branch will be slightly different than your `v2.0.0` tag since it will move forward with your new changes, so do be careful.