Added BUILD.md and a picture of the process.

BUILD.md

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+# How to turn these files into a book
+
+This repostitory contains a Makefile which you can use to turn a set of
+chapters written in [Markdown](https://daringfireball.net/projects/markdown/)
+into various other formats.
+
+In order to do that you will need various other tools installed. This file
+explains which tools are required, what they are required for and how to get
+them.
+
+## Make
+
+[Make](https://en.wikipedia.org/wiki/Make_%28software%29) is the standard Unix
+tool for controlling how to build files from other files. If you're using a
+system based on Unix (that includes Linux, BSD and OSX) then you will almost
+certainly alread have Make installed. If you're using Windows, then I think
+that Microsoft have a similar tool called `nmake` which you can almost
+certainly download from one of their many web sites.
+
+## Pandoc
+
+The first file that we need to generate is the
+[EPUB](https://en.wikipedia.org/wiki/EPUB) version of the book. And for that
+we use a tool called [Pandoc](http://pandoc.org/). This almost certainly
+won't already be installed on your system.
+
+For Linux users, Pandoc will be available in your system's package repository.
+If you're using a RedHat/Fedora/Centos system you can install it with `sudo
+install pandoc`. For a Debian/Ubuntu system, the equivalent command is `sudo
+apt-get install pandoc`. If you're using any other kind of Linux then I'm sure
+you're clever enough to work out the equivalent command.
+
+For OSX and Windows users, there are installable packages of the latest
+version of Pandoc available from their
+[Github repository](https://github.com/jgm/pandoc/releases/latest). Simply
+download the correct file and double-click it.
+
+Once you have Make and Pandoc installed, you can build an EPUB of the book by
+opening a terminal, changing to the directory where you have checked out a
+copy of this repository and typing:
+
+    make epub
+
+Or, even, just:
+
+    make
+
+You will end up with a file called perlwebbook.epub which can be used in many
+ebook systems.
+
+## Kindlegen
+
+If you want to generate a copy of the book which you can read on an Amazon
+Kindle, then you need to go a step further and generate a
+[MOBI](https://en.wikipedia.org/wiki/Mobipocket) version of the file. We do
+this using a program called Kindlegen which Amazon makes available for many
+different platforms. You can download the program
+[from their web site](http://www.amazon.com/gp/feature.html?docId=1000765211).
+
+Once you have installed Kindlegen, you can generate your MOBI file by opening
+a terminal, changing to the directory where you have checked out a copy of
+this repository and typing:
+
+    make mobi
+
+**Note:** The MOBI is generated by converting the EPUB. So running `make mobi`
+will automatically run `make epub` first, which means that you will also need
+to install Pandoc as described above.
+
+## Calibre
+
+Sometimes you don't want a real ebook version of the book. Sometimes just a
+[PDF](https://en.wikipedia.org/wiki/Portable_Document_Format) is enough. So
+our Make system supports building a PDF too.
+
+Building a PDF uses another tool called [Calibre](http://calibre-ebook.com/)
+which you will need to install if you want to make a PDF.
+
+For Linux users, Calibre will be available in your system's package repository. 
+If you're using a RedHat/Fedora/Centos system you can install it with `sudo
+install calibre`. For a Debian/Ubuntu system, the equivalent command is `sudo
+apt-get install calibre`. If you're using any other kind of Linux then I'm
+sure you're clever enough to work out the equivalent command,
+
+For OSX and Windows users, there are installable packages of the latest
+version of Calibre available from their
+[web site](http://calibre-ebook.com/download). Simply download the correct
+file and double-click it.
+
+Once you have Calibre installed, you can build a PDF of the book by opening
+a terminal, changing to the directory where you have checked out a copy of
+this repository and typing:
+
+    make pdf
+
+**Note:** The PDF is generated by converting the EPUB. So running `make mobi`
+will automatically run `make epub` first, which means that you will also need
+to install Pandoc as described above.
+
+## Conversion Process
+
+This section summarises the software needed to convert the input files into
+various output files.
+
+* This repository contains the book's sources in Markdown format.
+* An EPUB is created from the Markdown using Pandoc
+* A MOBI is created from the EPUB using Kindlegen
+* A PDF is created from the EPUB using Calibre
+
+Putting it all together, it looks like this (ebook_convert is the command
+line program that we use from the Calibre package):
+
+![Conversion Process](https://raw.githubusercontent.com/davorg/perlwebbook/master/converters.png)