Merge bitcoin#18663: doc: mention build docs in README.md

bda62e8 Adding build instructions to Bitcoin Core, fixes bitcoin#18658 (Saahil Shangle)

Pull request description:

  Making the instructions for building Bitcoin Core more clear in the main `README.md` will reduce confusion between the `build_msvc` and `doc` folders.

ACKs for top commit:
  laanwj:
    ACK bda62e8

Tree-SHA512: ee4c394661eba48d4229e3d1e9ddb67ccb79589429bfa9986cb0242cd615d1f3cc5332063562c1e89c0cdd9ae2e609f61e8bfb209926d8363d35d3da6d94ae9c

README.md

@@ -25,7 +25,7 @@ information or see https://opensource.org/licenses/MIT.
 Development Process
 -------------------
 
-The `master` branch is regularly built and tested, but is not guaranteed to be
+The `master` branch is regularly built (see doc/build-*.md for instructions) and tested, but is not guaranteed to be
 completely stable. [Tags](https://github.com/bitcoin/bitcoin/tags) are created
 regularly to indicate new official, stable release versions of Bitcoin Core.