docs: Add "Hello World" example

Let's get practical faster in the manual and have a simple "Hello World"
example right off the bat to hopefully make it easier to grok how OSTree
works.

Also some minor tweaks on wording around comparisons to git.

Closes: #1581
Approved by: cgwalters

README.md

@@ -20,9 +20,9 @@ bootloader configuration.
 
 The core OSTree model is like git in that it checksums individual files and has
 a content-addressed-object store. It's unlike git in that it "checks out" the
-files via hardlinks, and they should thus be immutable. Therefore, another way
-to think of OSTree is that it's just a more polished version
-of
+files via hardlinks, and they thus need to be immutable to prevent corruption.
+Therefore, another way to think of OSTree is that it's just a more polished
+version of
 [Linux VServer hardlinks](http://linux-vserver.org/index.php?title=util-vserver:Vhashify&oldid=2285).
 
 **Features:**