doc: Add note about STM32CubeIDE project being replaced with Makefile (#369)

Heads up for our users that we are about to remove the STM32CubeIDE
project from the ubxlib repo.

The message is embedded as HTML in readme_images/important_msg.svg.
GitHub have very restricted inline HTML support so there seems to be no
way of changing background color of things. But by using a .svg file and
the "foreignObject" it is possible.

Author: antevir

README.md

@@ -1,5 +1,7 @@
 ![u-blox logo](/readme_images/ublox-logo.png)
 
+![important message](/readme_images/important_msg.svg)
+
 # Introduction to `ubxlib`
 This repository contains the C code support library for [u-blox](https://www.u-blox.com) modules with [cellular](https://www.u-blox.com/en/cellular-modules) (2G/3G/4G), [short-range](https://www.u-blox.com/en/short-range-radio-chips-and-modules) (Bluetooth and Wi-Fi) and [positioning](https://www.u-blox.com/en/positioning-chips-and-modules) (GNSS) support. The library presents high level C APIs for use in customer applications (e.g. connect to a network, open a TCP socket, establish location, etc.) and implements these APIs on selected popular MCUs, also available inside u-blox modules.
 

port/platform/stm32cube/mcu/stm32f4/runner/README.md

@@ -1,4 +1,6 @@
 # Introduction
+> ⚠️ **IMPORTANT NOTE:** The STM32CubeIDE project will soon be replaced with a Makefile.
+
 This directory contains a build which compiles and runs any or all of the examples and tests for the STM32F4 MCU under the STM32Cube IDE.
 
 IMPORTANT: the STM32Cube IDE creates lots of sub-directories in here.  Please ignore them, NONE of the source files/header files are down here, they are all up in the `app` sub-directory for this platform so that they can be built with other/future ST SDKs as required.  Don't delete the empty directories though: if you do the STM32Cube IDE will then delete the links to the files in the `.project` file.