[readme] added the special build instruction for forked repo developer.

README.md

@@ -155,7 +155,7 @@ cd build
 cmake ../
 make
 ```
-
+For those working on a forked repository. If you forked with the ```Copy the main branch only``` option, then the repository will not include the necessary git tags to determine the Multipass version during CMake configuration. Therefore, you need to manually fetch the tags from the upstream by running ```git fetch --tags https://github.com/canonical/multipass.git``` in the ```<multipass>``` source code directory. 
 ## Running Multipass daemon and client
 
 First, install multipass's runtime dependencies. On amd64 architecture, you can achieve that with: