doc: linux host test documentation

examples/build_system/cmake/linux_host_app/README.md

@@ -1,8 +1,35 @@
 | Supported Targets | Linux |
 | ----------------- | ----- |
 
+This hello-world example builds a simple hello-world application for Linux.
+The compiler used is the Linux-gcc.
+
+There are two major differences to an IDF application built for an ESP chip compared to an application build for Linux:
+
+1. The entry-point on Linux is `int main(int argc, char **argv)`, instead of `void app_main(void)` on an ESP chip.
+   In this example for Linux, the `void app_main(void)` function is still included to make the connection to the IDF entry point clearer.
+   However, it is simply called by `int main(int argc, char **argv)`.
+   Refer to the source file [linux_host_app.cpp](main/linux_host_app.cpp) to see how it is used.
+
+2. The project-level [CMakeLists.txt](CMakeLists.txt) for Linux is different from that of a normal IDF application for an ESP chip.
+   On Linux, there is an additional line `set(COMPONENTS main)`, which clears the common requirements (default dependencies usually included in all IDF applications).
+   This is currently necessary as the Linux-host feature is still under developement.
+   Otherwise, a lot of hardware-dependent code would be pulled in.
+
 # Build
-`idf.py build` (sdkconfig.defaults sets the linux target by default)
+Source the IDF environment as usual, then set the Linux target:
+```bash
+idf.py --preview set-target linux
+```
+sdkconfig.defaults sets the Linux target by default, so this not strictly necessary.
+
+Once this is done, build the application:
+```bash
+idf.py build
+```
+Since this application runs on host, the flashing step is unnecessary.
 
 # Run
+```bash
 `build/linux_host_app.elf`
+```