Dependence on 'lunar' library was not noted. Now it is. Thanks to you…

…jin2 for pointing this out (see issue #1).

README.md

@@ -1,7 +1,12 @@
 # sat_code
 
 C/C++ code for the SGP4/SDP4 satellite motion model,  and for manipulating TLEs
-(Two-Line Elements). Full details at http://www.projectpluto.com/sat_code.htm
+(Two-Line Elements). Full details at
+[http://www.projectpluto.com/sat_code.htm](http://www.projectpluto.com/sat_code.htm).
+
+The only dependency is on the [`lunar`](https://github.com/Bill-Gray/lunar) (basic
+astronomical ephemeris/time functions) library.  Make and `make install` that
+library before attempting to build this code.
 
 On Linux,  run `make` to build the library and various test executables.
 (You can also do this with MinGW under Windows.)  In Linux,  you