readme: Omit phrasing; 'new'

This commit removes the word "new" in reference to describing Bitcoin,
as it has been around for over seven years, now.

README.md

@@ -8,7 +8,7 @@ https://bitcoincore.org
 What is Bitcoin?
 ----------------
 
-Bitcoin is an experimental new digital currency that enables instant payments to
+Bitcoin is an experimental digital currency that enables instant payments to
 anyone, anywhere in the world. Bitcoin uses peer-to-peer technology to operate
 with no central authority: managing transactions and issuing money are carried
 out collectively by the network. Bitcoin Core is the name of open source