Add section on build requirements to the README

I'm not sure that this is section is complete (may be missing
requirements), or accurate (minimum automake or bison versions
may not be correct).  However, I think it's useful, and we can
adjust it in the future to add requirements or adjust
requirement versions.

Fixes [Bug #14409]

README.md

@@ -76,6 +76,18 @@ in the mail body (not subject) to the address [ruby-talk-request@ruby-lang.org].
 
 [ruby-talk-request@ruby-lang.org]: mailto:ruby-talk-request@ruby-lang.org?subject=Join%20Ruby%20Mailing%20List&body=subscribe
 
+## Requirements to build from repository
+
+1. GNU or BSD make
+2. C99 compiler
+3. autoconf 2.67 or higher
+4. automake 1.15 or higher
+5. bison 2.3 or higher
+6. Ruby 2.2 or higher
+
+When building from a released version, only a C99 compiler and GNU or BSD make
+is required.
+
 ## How to compile and install
 
 1.  If you want to use Microsoft Visual C++ to compile Ruby, read