Document the asciidoc dependency

Author: lenormf

README.asciidoc

@@ -102,6 +102,7 @@ Kakoune dependencies are:
  * A C++11 compliant compiler (GCC >= 4.8 or clang >= 3.4)
  * boost (>= 1.50)
  * ncurses with wide-characters support (>= 5.3, generally referred to as libncursesw)
+ * asciidoc (for the `a2k` tool), to generate man pages
 
 To build, just type *make* in the src directory