Create separate page for general info about adding service brokers

- populate it with content from the github service broker readme and from writing-service.html
- add links on architecture/services/index to the new "adding service brokers" page and the example (github) service broker
- include both cf and gcf commands on these pages
add "developing a service broker" section to services/index page
Add "Developing a service broker" section on services index page

and add sections about listing and removing brokers to "managing service brokers" page
Add anchors to new headings in services/index.html

- minor edits in writing-service.html
add changes from PR #236