Bitcoin has now been around for over 13 years and has reached a market capitalization of $1 Trillion+ USD. It is now legal tender in El Salvador and is used by numerous publically listed companies as a treasury asset (https://bitcointreasuries.net/). The 2017 Blocksize wars and subsequent forks show that Bitcoin has now moved past being experimental.
This PR removes the phrase experimental from the opening paragraph of the README, changing "Bitcoin is an experimental digital currency" to "Bitcoin is a digital currency".