drl2gp: added README and a copy of the GPLv2

drl2gp/README

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+Drill file (Excellon) to gnuplot conversion
+===========================================
+
+drl2gp converts drill instructions in KiCad-generated Excellon files
+to tool paths in gnuplot notation.
+
+drl2gp should be integrated into "cameo" (../cameo) at some point in
+time.
+
+This work is distributed under the terms of the GNU GENERAL PUBLIC
+LICENSE, Version 2:
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation; either version 2 of the License, or
+  (at your option) any later version.
+
+For your convenience, a copy of the complete license has been included
+in the file COPYING.GPLv2.
+
+
+Usage
+-----
+
+drl2gp reads a drill file and generates toolpaths for a specific tool.
+Instructions that cannot be executed with the specified tool, e.g.,
+holes smaller than the tool's diameter, are ignored. 
+
+To facilitate iterative generation of toolpaths, drl2gp can generate a
+copy of its input file from which the instructions it has turned into
+toolpaths are removed. A typical workflow would then consist of a
+series drl2gp runs with drill, followed of a run with an endmill.
+
+drl2gp adjusts toolpath for the tool diameter. It is therefore not
+necessary to run "cameo" on the toolpaths.
+
+Usage:
+
+  drl2gp depth drill_d_min drill_d_max <file.drl >toolpaths.gp
+  drl2gp depth mill_d <file.drl >toolpaths.gp
+
+"depth" is the depth to which to drill. The tool can either be a drill
+or an endmill. A drill makes holes efficiently but cannot be moved
+sideways. An endmill can be used to cut holes wider than the tool
+diameter and it can cut slots.
+
+For a drill, the minimum and the maximum diameter of holes it should be
+used for is specified. For an endmill, the tool diameter is specified.
+
+All dimensions are in millimeters by default. The unit can be set to mil
+with the option -i and set back to millimeters with -m. -i and -m can be
+repeated any number of times and affect the parameters following them.
+
+If the option  -f out.drl  is given, drl2gp writes a copy of the
+Excellon file from which the drill instructions that have been accepted
+in this run have been stripped.
+
+Example:
+
+  drl2gp 1 -i 10 15 <file.drl -f tmp.drl >drill-135.gp
+  drl2gp 1 0.8 <tmp.drl >mill-08.gp
+
+Here, first generate toolpaths for a 13.5 mil drill. We use this drill
+for all holes with a diameter from 10 to 15 mil. Second, we use a 0.8
+mm endmill for the remaining drill instructions.
+
+
+Known bugs
+----------
+
+- most of the elements in an Excellon file are just ignored, without
+  examining if some may have an effect
+
+- when encountering a slotting operation with a compatible tool diameter,
+  drl2gp tries to process it even if the tool is a drill. This gets
+  detected when trying to generate the toolpath and results in an
+  assertion failure.
+
+  As work-around, choose the endmill in a set of tools such that it is
+  suitable for all slots, and only use drills smaller than the smallest
+  slot. (drl2gp will then use the endmill for everything that's left
+  after the drills.)
+
+- drl2gp should warn if any drill instructions are left for which no
+  toolpath could be generated (if not using -f)