Add instructions for contributing patches.

Adds step-by-step instructions for contribution to README.md:

Getting Started

1. Clone this repo:
   • git clone https://github.com/CrisisTextLine/CrisisTrends
2. Install Gulp and other depends:
   • npm install -g gulp
   • npm install
3. Start gulp server:
   • gulp serve

Start Coding

1. Fork this repository.
2. Clone the new fork:
   • git clone https://github.com/$YOUR-GITHUB-USER-NAME/CrisisTrends
3. Set up depends:
   • npm install -g gulp
   • npm install
4. Create a branch for your bugfix/feature/etc
   • git checkout branch fix-bug-name
   • now write your patch.
5. Check out your changes:
   • gulp test
   • gulp serve
6. Commit and push your changes to your repo:
   • git commit $FILE-YOU-CHANGED -m "fixes widget bug #0000
   • git push
   • If your changes are ready, test them and submit your pull request.

I used more or less the method I use myself for development in my fork of the repo. The prior instructions were lacking, and it still has no CONTRIBUTING.md, but that will be a separate issue.