update File Extensions guide and rename to Dependencies

Dependencies.md

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+# Guide to File Extensions
+
+A list of the file types you may encounter in this project. We have attempted to restrict the software dependencies to free programs as much as possible, with a few exceptions (and we are working on those).
+
+-  ```.sch``` [Eagle](http://www.cadsoftusa.com/eagle-pcb-design-software/?language=en) circuit schematic
+-  ```.brd``` [Eagle](http://www.cadsoftusa.com/eagle-pcb-design-software/?language=en) printed circuit board file
+-  ```.ods``` Open Document Spreadsheet file (supported by Excel, LibreOffice, etc.)
+-  ```.stl``` 3D part file that can be used to send parts to a 3D printer or production service
+-  ```.scad/escad``` [OpenSCAD](http://www.openscad.org)/[extended OpenSCAD](http://www.implicitcad.org) code used to generate 3D part files
+-  ```.part``` Proprietary [PTC Creo](http://www.ptc.com/product/creo/) CAD file. We try to provide open source equivalents to these files as much as possible, because CAD software can be very expensive.
+-  ```.md ``` [Markdown](https://help.github.com/articles/markdown-basics) file. Just a text file with standardized syntax for headings and things. 
+-  ```.ino``` [Arduino](http://www.arduino.cc) program
+-  ```.c/.h/.pde``` C source files for Arduino libraries
+
+**See also:** [Parts List](hardware/Parts List.tsv)