document "non-system compilers"

[Paul Hargrove]

Currently README.md includes the following text:

• Linux/x86-64 with gcc-5.1.0 or newer, or with clang-3.7.0 when using libstdc++ from gcc-5.1.0 (but see the note [to be provided] about use of non-system compilers).

This issue is to ensure I don't forget that I am responsible for the "to be provided" note.

[Paul Hargrove]

To ease collection of feedback, I have drafted this documentation in Google Docs and given Comment permissions to the Pagoda group. Once the text is stable I'll convert to Markdown and file a PR for it's inclusion.

cc: @bonachea

[Paul Hargrove]

I've opened Pull Request #3 with the proposed new file docs/local-gcc.md

[Paul Hargrove]

Document use of a locally-built gcc on Linux

Resolves issue #47

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