Update Readme.md

[st3b1t]

some details in readme.
minor versions affected by this

[RCasatta]

Thanks!

[smartm0use]

some details in readme. minor versions affected by this

Even with rustc = 1.65.0 compiling is affected by that issue:

[...]
error[E0658]: use of unstable library feature 'map_first_last'
   --> src/threads/update_mempool_info.rs:256:38
    |
256 |         mempool_fees.highest = rates.last().map(Into::into);
    |                                      ^^^^
    |
    = note: see issue #62924 <https://github.com/rust-lang/rust/issues/62924> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `fbbe` due to previous error
warning: build failed, waiting for other jobs to finish...