Fix clone instructions in README.md: use https instead of ssh

[trson]

Please follow the guide below

• You will be asked some questions and requested to provide some information, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your issue (like that [x])
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Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2017.02.14. If it's not read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2017.02.14

Before submitting an issue make sure you have:

• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

Description of your issue, suggested solution and other information

In step 2. of the "Adding support for a new site" section of README.md, the suggested command line for cloning the newly forked repository fails with the current default configuration of github repositories:

git clone git@github.com:YOUR_GITHUB_USERNAME/youtube-dl.git

I get the error:

Permission denied (publickey).
fatal: Could not read from remote repository.

One should clone the repo using https instead:

git clone https://github.com/YOUR_GITHUB_USERNAME/youtube-dl.git

[yan12125]

In the section "Adding support for a new site", it's assumed that people comes here for writing codes and open pull requests. To do that you need a working Github account. If you got permission denied, there's something wrong in your github account settings.