Add documentation

[kmelmon]

Document each sample with a detailed DOCS.md file.
a. WinForms
i. MyApp, how and where do I create it
ii. How does keyboard input works
b. WPF
i. MyApp, how and where do I create it
ii. How does keyboard input works
c. C++
i. MyApp, how and where do I create it
ii. Message pump
iii. How TABing works
iv. How does keyboard input works
v. How does Xaml accelerators and Win32 accelerators work together.