Non-SSL/TLS Links in README Building Section Should Use SSL/TLS

[U8NWXD]

Prerequisites

• Put an X between the brackets on this line if you have done all of the following:
  • Reproduced the problem in Safe Mode: http://flight-manual.atom.io/hacking-atom/sections/debugging/#using-safe-mode
  • Followed all applicable steps in the debugging guide: http://flight-manual.atom.io/hacking-atom/sections/debugging/
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Description

The links under the Building header in the README point to the http:// versions of sites even though the sites support SSL/TLS. Many other links in the README point to https:// sites, and I think these should too.

Steps to Reproduce

1. Open the README file in the core repository
2. Navigate to the Building section
3. Follow the link my clicking on it or copying it into a browser

Expected behavior: The link leads to an SSL/TLS-secured site

Actual behavior: The link leads to a site without SSL/TLS

Reproduces how often: 100% of the time

Versions

The current version of the README on GitHub

Additional Information

Specifically, the links for Linux, macOS, and Windows are affected. I have not really contributed to open source projects before, but I am interested in learning by fixing this issue.

The specific code I am referencing is copied below (copied from README.md in atom/atom on GitHub; copyrighted by GitHub Inc.):

## Building

* [FreeBSD](./docs/build-instructions/freebsd.md)
* [Linux](http://flight-manual.atom.io/hacking-atom/sections/hacking-on-atom-core/#platform-linux)
* [macOS](http://flight-manual.atom.io/hacking-atom/sections/hacking-on-atom-core/#platform-mac)
* [Windows](http://flight-manual.atom.io/hacking-atom/sections/hacking-on-atom-core/#platform-windows)

[U8NWXD]

Does this change make sense, or is there a reason to keep these links as http:// ones? If this is a change that makes sense, I'd be happy to submit a PR for it.

[winstliu]

Definitely makes sense. PRs would be appreciated.