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

@@ -37,6 +37,9 @@ The `master` branch is meant to be stable. Development is normally done in separ
 [Tags](https://github.com/dashpay/dash/tags) are created to indicate new official,
 stable release versions of Dash Core.
 
+The `develop` branch is regularly built (see doc/build-*.md for instructions) and tested, but is not guaranteed to be
+completely stable.
+
 The contribution workflow is described in [CONTRIBUTING.md](CONTRIBUTING.md)
 and useful hints for developers can be found in [doc/developer-notes.md](doc/developer-notes.md).