Added first draft of conventions. Sign for iota might be wrong

src/conventions.txt

+Lattitude is measured north of the equator.  So Lattitude -10 means 10
+degrees SOUTH of the equator.
+
+Longitude is measured East of Greenwich.  So longitude -90 means 90
+degrees West of Greenwich (somewhere in the USA).
+
+x,y,z coordinate system for Earth is as follows:
+
+(a) The equator lies in the XY plane
+(b) The north pole lies on the positive Z axis
+(c) The line of longitude 0 lies in the XZ plane and passes through
+    the positive X axis
+
+#THE FOLLOWING STATEMENT MAY NEED A SIGN CHANGE
+The orbital inclination angle iota (from 0 to 180 degrees) is the angle
+between the orbital angular momentum vector (defined with the
+right-hand rule) and the vector from earth to source.
+
+Face-on is iota=0 and face-off is iota=180.
+
+The orientation angle psi is the angle (from 0 to 180 degrees,
+measured counterclockwise from North) between the long angle of the
+ellipse (projected into the plane of the sky) and the northerly
+direction.
+