docs: Update contribution to use Gitflow workflow

[anatolyyyyyy]

Fixes #78

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

If you have questions, please send an email to Sendgrid, or file a Github Issue in this repository.

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #79 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #79   +/-   ##
=======================================
  Coverage   99.32%   99.32%           
=======================================
  Files           2        2           
  Lines         149      149           
=======================================
  Hits          148      148           
  Misses          1        1

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[hugovk]

Use plain language:

Suggested change

	Please note that we utilize the [Gitflow Workflow](https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow) for Git to help keep project development organized and consistent.
	Please note that we use the [Gitflow Workflow](https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow) for Git to help keep project development organized and consistent.

[anatolyyyyyy]

Good idea, done

[hugovk]

Suggested change

	- [ ] I updated my branch with the development branch.
	- [ ] I updated my branch with the `development` branch.

[anatolyyyyyy]

Updated

[thinkingserious]

Hello @anatolyyyyyy,

Thanks again for the PR!

It's HACKTOBERFEST! We want to show our appreciation by sending you some special Hacktoberfest swag. If you have not already, could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX

[thinkingserious]

Hello @anatolyyyyyy,

Thanks again for the PR!

You have earned 6 out of the 5 points needed to receive glorious SendGrid Hacktoberfest swag.

Please take a moment to checkout this link to find more issues to get you past the required threshold or to simply continue the celebration.

Also, please be sure you have officially registered with us here by November 1, 2018 to qualify.

If you have any questions you can email us at [email protected].

Thank you and Happy Hacktobering!

Team SendGrid