docs/man -> docs/src and update Makefile and build-helper.

docs/src/cdist-why.rst

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+Why should I use cdist?
+=======================
+
+There are several motivations to use cdist, these
+are probably the most popular ones.
+
+Known language
+--------------
+
+Cdist is being configured in
+`shell script <https://en.wikipedia.org/wiki/Shell_script>`_.
+Shell script is used by UNIX system engineers for decades.
+So when cdist is introduced, your staff does not need to learn a new
+`DSL <https://en.wikipedia.org/wiki/Domain-specific_language>`_
+or programming language.
+
+Powerful language
+-----------------
+
+Not only is shell scripting widely known by system engineers,
+but it is also a very powerful language. Here are some features
+which make daily work easy:
+
+ * Configuration can react dynamicly on explored values
+ * High level string manipulation (using sed, awk, grep)
+ * Conditional support (**if, case**)
+ * Loop support (**for, while**)
+ * Support for dependencies between cdist types
+
+More than shell scripting
+-------------------------
+
+If you compare regular shell scripting with cdist, there is one major
+difference: When using cdist types,
+the results are
+`idempotent <https://en.wikipedia.org/wiki/Idempotence>`_.
+In practise that means it does not matter in which order you
+call cdist types, the result is always the same.
+
+Zero dependency configuration management
+----------------------------------------
+
+Cdist requires very litte on a target system. Even better,
+in almost all cases all dependencies are usually fulfilled.
+Cdist does not require an agent or a high level programming
+languages on the target host: it will run on any host that
+has a **ssh server running** and a posix compatible shell
+(**/bin/sh**). Compared to other configuration management systems,
+it does not require to open up an additional port.
+
+Push based distribution
+-----------------------
+
+Cdist uses the push based model for configuration. In this
+scenario, one (or more) computers connect the target hosts
+and apply the configuration. That way the source host has
+very little requirements: Cdist can even run on a sysadmin
+notebook that is loosely connected to the network and has
+limited amount of resources.
+
+Furthermore, from a security point of view, only one machine
+needs access to the target hosts. No target hosts will ever
+need to connect back to the source host, which contains the
+full configuration.
+
+Highly scalable
+---------------
+
+If at some point you manage more hosts than can be handled from
+a single source host, you can simply add more resources: Either
+add more cores to one host or add hosts.
+Cdist will utilise the given resources in parallel.