Update syl20bnr readme
Alternate key binding for isearch-repeat-forward

By default it is on C-s
This commit adds S-return which consistent on more keyboard factor form
(for instance MacBook Air has not Control an the right side which makes
C-s hard to trigger).

This allows for a nice worklow to quickly search for text in a buffer:
1) initiate search with /
2) incremental search
3) look for candidates with S-return without closing isearch
4) loop on 2) and 3)
5) commit search with `return` or abort with `fd`