moving docs to docs/

docs/usage.md

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+# Usage
+
+The genplotting library use, as a backend, an external 
+[software](https://git.tetalab.org/tTh/libtthimage/src/branch/master/Tools/genplot2.c)
+who can be installed with the [libtthimage](https://git.tetalab.org/tTh/libtthimage) and convert scratchfiles to
+[Targa](https://en.wikipedia.org/wiki/Truevision_TGA) image file. 
+
+Three steps : initialize, use and terminating.
+
+## Initialize
+
+Call the subroutine `genp_init` with two parameters.
+The first is an integer and must be 0. 
+The second is the filename for recording the plottings instructions,
+if this filename is an empty string, *stdout* will be used.
+
+## Use it
+
+- real xa, ya, xb, yb
+- integer color
+- call genp_move(xa, ya)
+- call genp_draw(xa, ya, color)
+- call genp_line(xa, ya, xb, yb, color)
+
+## Terminator
+
+- call genp_end(0)
+
+
+