Update README.md

[lemz90]

Pull requests without a rationale and clear improvement may be closed
immediately.

Please provide clear motivation for your patch and explain how it improves
Bitcoin Core user experience or Bitcoin Core developer experience
significantly.

• Any test improvements or new tests that improve coverage are always welcome.
• All other changes should have accompanying unit tests (see src/test/) or
  functional tests (see test/). Contributors should note which tests cover
  modified code. If no tests exist for a region of modified code, new tests
  should accompany the change.
• Bug fixes are most welcome when they come with steps to reproduce or an
  explanation of the potential issue as well as reasoning for the way the bug
  was fixed.
• Features are welcome, but might be rejected due to design or scope issues.
  If a feature is based on a lot of dependencies, contributors should first
  consider building the system outside of Bitcoin Core, if possible.
• Refactoring changes are only accepted if they are required for a feature or
  bug fix or otherwise improve developer experience significantly. For example,
  most "code style" refactoring changes require a thorough explanation why they
  are useful, what downsides they have and why they significantly improve
  developer experience or avoid serious programming bugs. Note that code style
  is often a subjective matter. Unless they are explicitly mentioned to be
  preferred in the developer notes, stylistic code
  changes are usually rejected.

Bitcoin Core has a thorough review process and even the most trivial change
needs to pass a lot of eyes and requires non-zero or even substantial time
effort to review. There is a huge lack of active reviewers on the project, so
patches often sit for a long time.