README: Change markdown title formatting
README: Change markdown title formatting

Use the underlining style for headers, because it looks better when the
file is read in text format.

Refs SHOOP-987

drayanaindra/shoop,jacobsenanaizabel/shoop,janusnic/shoop,hrayr-artunyan/shuup,hrayr-artunyan/shuup,suutari/shoop,janusnic/shoop,jacobsenanaizabel/shoop,taedori81/shoop,shoopio/shoop,arth-co/shoop,suutari-ai/shoop,shawnadelic/shuup,jacobsenanaizabel/shoop,shawnadelic/shuup,shawnadelic/shuup,akx/shoop,taedori81/shoop,drayanaindra/shoop,drayanaindra/shoop,arth-co/shoop,jaywink/shoop,lawzou/shoop,shoopio/shoop,jorge-marques/shoop,jaywink/shoop,akx/shoop,lawzou/shoop,akx/shoop,shoopio/shoop,jaywink/shoop,hrayr-artunyan/shuup,suutari-ai/shoop,taedori81/shoop,jorge-marques/shoop,janusnic/shoop,suutari/shoop,lawzou/shoop,arth-co/shoop,jorge-marques/shoop,suutari/shoop,suutari-ai/shoop