Update FIPS instructions in README.md

Update the instructions to follow the requirements
in the security policy and user guide

PR-URL: nodejs#2278
Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Fedor Indutny <fedor@indutny.com>

2015-08-04 io.js v3.0.0 Release

2015-08-03 io.js v3.0.0 RC 7

2015-07-30 io.js v3.0.0 RC 6

2015-07-28 io.js v2.5.0 Release

2015-07-24 io.js v3.0.0 RC 5

2015-07-22 io.js v3.0.0 RC 4

doc: Add notes for using @@toStringTag

2015-07-17 io.js v2.4.0 Release

2015-07-12 io.js v3.0.0 RC 3