Update linux.md

Add step for checking out the latest release by git tag.
That is strange. It works for me with Git 1.9.3:

```
$ git status
HEAD detached at v0.119.0
nothing to commit, working directory clean
$ git rev-list --tags --max-count=1
736952d0a22eba922c5c04baac2876ef7e806315
```

Works on master, too:

```
$ git checkout master
[...]
$ git status
On branch master
Your branch is up-to-date with 'origin/master'.

nothing to commit, working directory clean
$ git rev-list --tags --max-count=1
736952d0a22eba922c5c04baac2876ef7e806315
$
```

[The manpage of git describe](http://linux.die.net/man/1/git-describe) states that it seeks for the tag in the ancestor commits or the given/current commit itself. So instead of passing the commit id of the latest tag using rev-list, one can just use the master branch. The HEAD of the master branch is always either a tagged commit or a descendant of the last tagged commit.

```
$ git status
On branch master
Your branch is up-to-date with 'origin/master'.

nothing to commit, working directory clean
$ git describe --tags --abbrev=0
v0.119.0
$ git checkout $(git describe --tags --abbrev=0)
Note: checking out 'v0.119.0'.

[...]

HEAD is now at 736952d... Upgrade to language-c@0.26
$ git status
HEAD detached at v0.119.0
nothing to commit, working directory clean
$ git checkout $(git describe --tags --abbrev=0 master)
Previous HEAD position was c03d34f... Upgrade to language-c@0.25
HEAD is now at 736952d... Upgrade to language-c@0.26
$
```