Add table of contents.
Add table of contents.

I used [DocToc](https://github.com/thlorenz/doctoc) to generate these.

    npm install -g doctoc
    doctoc README.md --github

It may or may not be the tool we use elsewhere, but we should use
something to maintain table of contents more automatically.

sduskis/cloud-bigtable-examples,GoogleCloudPlatform/cloud-bigtable-examples,sduskis/cloud-bigtable-examples,sduskis/cloud-bigtable-examples,sduskis/cloud-bigtable-examples,GoogleCloudPlatform/cloud-bigtable-examples,sduskis/cloud-bigtable-examples,sduskis/cloud-bigtable-examples,GoogleCloudPlatform/cloud-bigtable-examples,sduskis/cloud-bigtable-examples,GoogleCloudPlatform/cloud-bigtable-examples