Small README.md updates + a flaky_tests file

[pjaaskel]

No description provided.

[linehill]

Should not be needed. hipcc looks for $HIP_PATH/lib/.hipInfo which defines HIP_PLATFORM=spirv and other important variables.

(If HIP_PATH is not defined it defaults to <path-to-hipcc>/..).

[franz]

@pjaaskel I've added your README changes to my PR. Please push directly into that branch (& notify me) if you have more documentation updates. WRT the issue not requiring both backends, there's a fix in that PR. WRT the flaky tests, cat ../flaky_tests assumes that the build is subdir of source. I've instead added a custom target named "check" (like in pocl) that skips failing tests with a regexp. If you run into more failing tests, please add them to the regexp (i'll add some more too).