Docs on clang-tidy #70

Add docs to the extended section on:

• What is clang-tidy?
• What kind of bugs does it catch?
• How to run it locally: make tidy
• When you see a failure, what do you do? (hint: clang-tidy outputs shorthand codes for each error/warning that can be googled to get to its individual page like https://clang.llvm.org/extra/clang-tidy/checks/performance-unnecessary-copy-initialization.html)
• Add note that when forward porting node-cpp-skel to another existing node c++ addon, it is expected that you will hit clang-tidy errors. Fixing them can take some time, so some devs may wish to disable the travis job to be able to triage slowly.

Context: clang-tidy support landed in #64