README edits, fig install, site update

Makefile

@@ -236,6 +236,9 @@ ifneq ($(wildcard apidoc/html/search/*),)
 	install -d $(DESTDIR)$(htmldir)/search
 	$(INSTALL_DATA) -D apidoc/html/search/* $(DESTDIR)$(htmldir)/search/
 	$(INSTALL_DATA) -D apidoc/html/*.* $(DESTDIR)$(htmldir)/
+	@echo "installing images to $(DESTDIR)$(htmldir)/resources"
+	install -d $(DESTDIR)$(htmldir)/resources
+	$(INSTALL_DATA) -D resources/SupeNOVAS-systems.png $(DESTDIR)$(htmldir)/resources/
 	@echo "installing Doxygen tag file to $(DESTDIR)$(docdir)"
 	install -d $(DESTDIR)$(docdir)
 	$(INSTALL_DATA) apidoc/supernovas.tag $(DESTDIR)$(docdir)/supernovas.tag

README.md

@@ -402,6 +402,9 @@ In NOVAS, the barycentric BCRS and the geocentric GCRS systems are effectively s
 for positions and for velocities, in any reference system, is determined by the `observer` location, while aberration
 and gravitational deflection is included for apparent places only (as seen from the observer location). 
 
+Older catalogs, such as B1950 (FK4) or B1900 are just special cases of MOD (mean-of-date) coordinates for the B1950
+and B1900 epochs, respectively.
+
 TIRS (Intermediate Terrestrial Reference System) is not explicitly referenced in SuperNOVAS. But it can be thought 
 of as a special case of ITRS (International Terrestrial Reference System) with DUT1 (= UT1-UTC), the polar offsets 
 (_dx_, _dy_) all set to zero.

src/novas.c

@@ -4506,6 +4506,9 @@ double rad_vel2(const object *source, const double *pos_emit, const double *vel_
  * @sa tt2tdb()
  * @sa cio_basis()
  * @sa NOVAS_TOD
+ * @sa NOVAS_JD_J2000
+ * @sa NOVAS_JD_B1950
+ * @sa NOVAS_JD_B1900
  */
 short precession(double jd_tdb_in, const double *in, double jd_tdb_out, double *out) {
   static THREAD_LOCAL double t_last;