Make easy targets section more readable.

Created by: PeterZhizhin

Problem: [describe the problem you try to solve with this PR.] For me (and as I think, for many others) it was very difficult to distinguish tasks that are proposed for Rust fluent developers (as well as OS and non-OS experts) and Rust newbies.

Solution: [describe carefully what you change by this PR.] Add subsections for each type of developers.

Changes introduced by this pull request:

• Added subsections in "Low-hanging fruit" section.