[shootout] add initial docs for n-body example
[shootout] add initial docs for n-body example

perl6/perl6-examples,perl6/perl6-examples,Quinny/perl6-examples,stmuk/perl6-examples,perl6/perl6-examples,perl6/perl6-examples,shlomif/perl6-examples,shlomif/perl6-examples,stmuk/perl6-examples,Quinny/perl6-examples,shlomif/perl6-examples,shlomif/perl6-examples,shlomif/perl6-examples,stmuk/perl6-examples,Quinny/perl6-examples,stmuk/perl6-examples,Quinny/perl6-examples,stmuk/perl6-examples,Quinny/perl6-examples,perl6/perl6-examples