Add page about systick and tickless mode

[bulislaw]

CC: @AnotherButler @c1728p9 @0xc0170

[AnotherButler]

Queries:

• Are these APIs or just modes?
• It says these should be invisible to users. Does that mean mostly vendors will be using them? Should this go into porting?

[AnotherButler]

@bulislaw I've added some copy edits. Please review my changes, and make sure I didn't accidentally change the content.

[bulislaw]

Are these APIs or just modes?
It says these should be invisible to users. Does that mean mostly vendors will be using them?Should this go into porting?

They are features really, but you are probably right, I wanted to put somewhere the description of how Mbed OS works. Considering it's more useful from the vendor's point of view I'll move it to the porting guide (and rebase on top of the new engine branch).

[bulislaw]

@AnotherButler I've rebased the doc on top of the new_engine branch, moved it to the contributing directory and added some info that make it part of the PG.

[AnotherButler]

@bulislaw Nice work. I've added some minor copy edits.

@0xc0170 Please review, and let me know when I can merge.

[AnotherButler]

Thanks for the quick review, @c1728p9 I'd forgotten both @bulislaw and @0xc0170 were on holiday.