docs: Add FreeBSD build notes link to doc/README.md

Author: fanquake

doc/README.md

@@ -41,9 +41,10 @@ The following are developer notes on how to build Bitcoin Core on your native pl
 - [macOS Build Notes](build-osx.md)
 - [Unix Build Notes](build-unix.md)
 - [Windows Build Notes](build-windows.md)
+- [FreeBSD Build Notes](build-freebsd.md)
 - [OpenBSD Build Notes](build-openbsd.md)
 - [NetBSD Build Notes](build-netbsd.md)
-- [Gitian Building Guide](gitian-building.md)
+- [Gitian Building Guide (External Link)](https://github.com/bitcoin-core/docs/blob/master/gitian-building.md)
 
 Development
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