freetype2: Fix comments in freetype.hpp and README.md

[Kumataro]

Fix #2771 and related small problems about documentation.

1. freetype.hpp is fixed.

• putText() is supported for only 8UC3 image with only 8UC3 color.

2. README.md is fixed.

• Use correct markdown format.
• Change document title from "freetype module" to "freetype wrapper module".
  • This module title was "freetype module".
  • FText primitive in GAPI uses `freetype library".
  • Generally "module" and "library" in the computer world are similar.
  • So that, I think this module title should be changed to not confuse it.
• Installation is replaced as Requested external libraries
• Usage is update to it can be compile in real.
• Future work is removed.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[alalek]

Thank you for contribution.

Please take a look on comments below.

[alalek]

8bit

Scalar values are doubles by design.
I believe we should mention used range, like 0-255, here instead.

[Kumataro]

Thank you very much!

Because freetype2 wrapper module requests that img is 8UC3, I reconsider this limitation of color is very clear.

• first 3 channels of 'color' are used, 4th channel of it is not.
• The range of element are 0-255.

This change is not necessary. I cancel adding comment.

[alalek]

Please remove dots from the Headers

Please align number of === / --- symbols on the next line (as markdown requested)

[Kumataro]

Thank you very much ! I'll fix it.