Class 2 UIS threads

Documented that threadlib provides *class* *2* UIS threads,
not class 1 and not class 3. Clarifies issue #33.

README.rst

@@ -132,7 +132,7 @@ Currently, threadlib knows these threads:
 
 - `Metric threads <http://mdmetric.com/tech/M-thead%20600.htm>`__ (coarse, fine, and super-fine pitches) M0.25 to M600
 - Unified Inch Screw Threads (UNC, UNF, UNEF, 4-UN, 6-UN, 8-UN, 12-UN,
-  16-UN, 20-UN, 28-UN, and 32-UN)
+  16-UN, 20-UN, 28-UN, and 32-UN). All threads are class 2 threads.
 - `BSP parallel thread <https://www.amesweb.info/Screws/British-Standard-Pipe-Parallel-Thread-BSPP.aspx>`__ G1/16 to G6
 - `PCO-1881 <https://www.bevtech.org/assets/Committees/Packaging-Technology/20/3784253-20.pdf>`__ (PET-bottle thread)
 

design/UIS_thread.awk

@@ -8,6 +8,9 @@ function min(x, y) {
 }
 
 function calculateThreadlibSpecs() {
+	# Calculates threadlib specs for UIS threads assuming class 2
+	# threads (class 1 and class 3 threads are not supported)
+
 	Designator = $1;
 	TPI = $2;
 	D = $3;