doc: headerify the Install instructions
doc: headerify the Install instructions

Enable linking to the platform specific installation instructions

PR-URL: https://github.com/nodejs/node-gyp/pull/1225
Reviewed-By: Refael Ackermann <0604a90ccdde7a0adfa2ff3857349859a7209740@gmail.com>

nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodejs/node-gyp