Document the .mbedignore file

[SeppoTakalo]

Add documentation about the .mbedignore file.

Please wait for someone to check the language before merging.

[SeppoTakalo]

@bogdanm @screamerbg or @sg- Please review.

[screamerbg]

👍 LGTM

[peknis01]

Skip the first sentence. Instead start: The .mbedignore file allows you to ignore...

[0xc0170]

shall the doc name be mbed_ignore_file or might be better to have : build_system (ignore file would be part of the bigger document about our build system, to describe how we build our sources and how we can ignore, etc) rather than Ignoring_files_from_build (I would not capital I in the name, the other docs do not use it neither).

[SeppoTakalo]

Edited. Pekka's suggestions taken into use.

@0xc0170 I agree that this should be part of the build system documentation. However, the BUILDING.md documentation currently talks about testing. So the basic build system documentation is still missing. Or maybe that needs to be split into two parts, building and testing. Therefore I suggest that this goes in as a separate file, and we will integrate these later when forming the explanation of build system.

This is ready to be merged.