Add rgb documentation

doc/lush.txt

@@ -35,6 +35,7 @@ HSL & HSLuv Colors                         |lush-hsl-colors| |lush-hsluv-colors|
   abs_*                                         |lush-color-absolute-operations|
   hue, saturation, lightness, .h, .s, .l                     |lush-color-direct|
   .hex, tostring                                    |lush-color-string-coercion|
+  .rgb                                                          |lush-color-rgb|
   Example                                                   |lush-color-example|
 Lush Spec                                                            |lush-spec|
   Sample lush-spec                                            |lush-spec-sample|
@@ -205,9 +206,28 @@ And access members with,
 
 .hex, tostring                                      *lush-color-string-coercion*
 
-Finally, HSL colors can be coerced into a hex string, either by concatenation
-with another string, passing to a function which will invoke tostring() or
-accessing the `.hex` accessor.
+HSL colors can be coerced into a hex string, either by concatenation
+with another string, passing to a function which will invoke tostring()
+or accessing the `.hex` accessor.
+
+Example                                                     *lush-color-example*
+>
+  local hsl = require('lush').hsl                 -- include the module
+  local red = hsl(0, 100, 50)                     -- define a color
+  local light_red = red.lighten(20)               -- modify
+  local orange = red.hue(20)                      -- set
+  local sum_hues = red.h + light_red.h + orange.h -- access
+  local chained_compliment = red.ro(180)          -- chain via aliases
+                                .da(30)
+                                .sa(10)
+  print(red)                                      -- as string "#FF0000"
+<
+
+.rgb()                                                          *lush-color-rgb*
+
+HSL colors can be converted to RGB values, this should only be done
+outside of a Lush spec, when you are intending to extract the red, green
+and blue values for use elsewhere.
 
 Example                                                     *lush-color-example*
 >