Small clarification and md cleanup

[FranzEricSchneider]

1. Added the small "top-level gtsam directory" clarification because for a few minutes I thought it was saying to do this build process inside of the examples directory, which didn't work.
2. Cleaned up the ## because without the space between icon and text it does not make a heading as intended. What the README looks like on develop at the moment:

[ProfFan]

@FranzEricSchneider Thanks! Could you also add the proper headers to the code examples?

[FranzEricSchneider]

@ProfFan sorry, I'm not quite sure what you mean. Could you elaborate? What would make a proper header? And which examples do you want it added to?

[dellaert]

@ProfFan sorry, I'm not quite sure what you mean. Could you elaborate? What would make a proper header? And which examples do you want it added to?

I also don’t know what Fan means :-)

[ProfFan]

@FranzEricSchneider Sorry I mistook this PR to be in the gtsam.org repo :(

In #852 (comment) you talked about the examples missing the headers they require. I am saying that you could add them in this PR.