simpleperf: split README.md in doc directory.

Move README.md to doc directory, split inferno doc into inferno.md,
move pictures from scripts/inferno/ to doc/.
Improve the content about showing flamegraphs in README.md.

Bug: http://b/64123842
Test: No.
Change-Id: Ia3ebd0bbbda21fa816b94cd349c901b5c7713564
9 files changed