build: osx readme: note cli xcode needs to be deleted before install

upgrading an existing directory does not seem to work well

contrib/osx/README.md

@@ -28,7 +28,7 @@ Notes about compatibility with different macOS versions:
 We currently build the release binaries on macOS 10.14.6, and these seem to run on
 10.13 or newer.
 
-Before starting, you should install `brew`.
+Before starting, you should install [`brew`](https://brew.sh/).
 
 
 #### Notes about reproducibility
@@ -86,6 +86,7 @@ $ sudo xcodebuild -runFirstLaunch
 
 Install Xcode CLI tools:
 ```
+$ sudo rm -rf /Library/Developer/CommandLineTools
 $ shasum -a 256 "$HOME/Downloads/Command_Line_Tools_for_Xcode_11.3.1.dmg"
 1c4b477285641cca5313f456b712bf726aca8db77f38793420e1d451588673f9  /Users/vagrant/Downloads/Command_Line_Tools_for_Xcode_11.3.1.dmg
 $ hdiutil attach "$HOME/Downloads/Command_Line_Tools_for_Xcode_11.3.1.dmg"