Improve the tutorial that may be confusing

[ghost]

This patch updates the tutorial which can be confusing for users
This issue has been raised by the users from

#2950

I tried minimize the revision because I don't know exactly what the previous writer intended
and so almost all of the original contents have been preserved

[romanwerpachowski]

Thanks!

[romanwerpachowski]

Thanks, that's a great improvement.

[vslashg]

This looks good, but could you revert the changes adding a space between "GoogleTest" and "GoogleMock"? I know we're inconsistent with this spelling in places, but we prefer to spell these as one word.

[ghost]

@vslashg

As I modified googletest/README.md,
I couldn't avoid to modify other two files for consistency (README.md, googlemock/README.md).
(even though there are still some inconsistencies)

I know that it must be very cautious to revise the main title, and titles.
but for now, I uploaded a patch which includes those changes.

Please let me know if there are any parts that should remain not modified.