Created Contributing (markdown)

Contributing.md

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+# introduction
+The source code of the IFM is a bit different from other projects you might know. Historically I developed it literally in one single file. As this became very confusing, I splitted the parts up into the `src` directory and searched for a nice way to compile it into a single file in the end. Other single-file script solutions, like [Adminer](https://github.com/vrana/adminer), use custom compilers for their scripts, so I decided to do it in a similar way.
+
+## compiler
+The compiler script is written in PHP and does essentialy three things:
+* replace the includes with actual file content
+* concat PHP files while removing the first line (`<?php`)
+* add snippet to start the IFM
+Includes can be made by writing the filename enclosed by three at signs (`@@@`).
+
+## files
+Here is a short explanation of what does what:
+* `src/main.php`: IFM class, core of the IFM
+* `src/ifm.js`: IFM client application, written in JS
+* `src/ifmzip.php`: zip class used by the IFM
+* `src/style.css`: some custom css
+* `src/includes/*`: external sources like bootstrap etc.
+* `src/templates/*`: HTML templates