Add introduction to example subsections

[PicoCentauri]

It might be helpful to guide users a bit better in the six example sections:

• PCovR and KernelPCovR
• PCovC
• Feature and Sample Selection
• Regression
• Feature Reconstruction Measures
• Neighbors

Each README.rst only contains the title of the section and the examples. There is no explanation of what each section is about and what the user should expect here. Already a couple of lines would help a lot.