Documentation / readme.md update

We've had a few issues recently where it's not clear what resource agent should be used.

For example in ticket #626 I didn't even realise lio had been forked to lio-t and I'm wondering if people would perhaps benefit from an index with brief description of each of the agent types.

This could simply be added to the readme.md in the project - I'd be happy to to submit a PR for the ones that I use / know about if others also wanted to contribute?

For example:
# Resource Agents

This repository contains resource agents (RAs) compliant with the Open Cluster Framework (OCF) specification.
## Agents & Implementation Types

Detailed information on each of the agents can be found by calling meta-data against each script.
For example `/usr/lib/ocf/resource.d/heartbeat/iSCSILogicalUnit meta-data`

---
## Storage
#### iSCSILogicalUnit

Configures iSCSI LUNs of various provider types
##### lio

kernel iSCSI implementation that uses tcm_node.py to manage configuration - deprecated as of RHEL/CentOS 7 in favour of lio-t.
Requirements:
- Package `targetcli`
- RHEL5/6 derivative.
##### lio-t

kernel iSCSI implementation that uses targetcli to manage configuration.
Requirements:
- Package `targetcli`
- RHEL7 derivative or newer.
#### iSCSITarget

Configures iSCSI Targets of the same provider types as iSCSILogicalUnit

---
## Networking
#### IPaddr2

etc...

Start with the project's readme.md and review the proposed Resource Agents structure in this issue. Use the mentioned meta-data command and the examples for iSCSILogicalUnit, lio, lio-t, and iSCSITarget to identify the information to document. Done means the README provides a useful index with brief descriptions and implementation or requirement details for the relevant agents.

Written by the indexing model from the issue text.