Use Zeit Now to deploy doc builds from PRs (#344)

"Continuous documentation" on every push/PR made to the repository. 
Uses a npm `package.json` configuration to setup miniconda,
install the necessary packages, build the documentation pages and then 
deploy. Add instructions to Maintenance Guide.

Author: weiji14

MAINTENANCE.md

@@ -61,6 +61,15 @@ repository to avoid duplicating work across multiple repositories.
 If you find any problems with the test setup and deployment, please create issues and
 submit pull requests to that repository.
 
+## Continuous Documentation
+
+We use the [Zeit Now for Github integration](https://zeit.co/github) to preview changes
+made to our documentation website every time we make a commit in a pull request.
+The integration service has a configuration file `now.json`, with a list of options to
+change the default behaviour at https://zeit.co/docs/configuration.
+The actual script `package.json` is used by Zeit Now to install the necessary packages,
+build the documentation, copy the files to a 'public' folder and deploy that to the web,
+see https://zeit.co/docs/v2/build-step/?query=package.json#defining-a-build-script.
 
 ## Making a Release
 

now.json

@@ -0,0 +1,6 @@
+{
+  "github": {
+    "silent": true
+  },
+  "public": true
+}

package.json

@@ -0,0 +1,8 @@
+{
+  "scripts": {
+    "build:miniconda": "curl -o ~/miniconda.sh https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh && bash ~/miniconda.sh -b -p $HOME/miniconda",
+    "build:pygmt": "conda env create -f environment.yml && source activate pygmt && conda install -c conda-forge -c conda-forge/label/dev -y gmt==6.0.0rc4 && make install",
+    "build:docs": "source activate pygmt && cd doc && make all && mv _build/html ../public",
+    "build": "export PATH=$HOME/miniconda/bin:$PATH && npm run build:miniconda && npm run build:pygmt && npm run build:docs"
+  }
+}