doc: give more attention to Catalina issues doc

It's easy to miss the Catalina issues doc when reading the readme. Since
it can be a common issue among developers, it makes sense to give more
attention to it in the readme.

PR-URL: #2134
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Christian Clauss <cclauss@me.com>

README.md

@@ -37,10 +37,11 @@ Depending on your operating system, you will need to install:
 
 ### On macOS
 
+**ATTENTION**: If your Mac has been _upgraded_ to macOS Catalina (10.15), please read [macOS_Catalina.md](macOS_Catalina.md).
+
    * Python v2.7, v3.5, v3.6, v3.7, or v3.8
    * [Xcode](https://developer.apple.com/xcode/download/)
      * You also need to install the `XCode Command Line Tools` by running `xcode-select --install`. Alternatively, if you already have the full Xcode installed, you can find them under the menu `Xcode -> Open Developer Tool -> More Developer Tools...`. This step will install `clang`, `clang++`, and `make`.
-   * If your Mac has been _upgraded_ to macOS Catalina (10.15), please read [macOS_Catalina.md](macOS_Catalina.md).
 
 ### On Windows