Docs update for tables support, snmpv3 example, and typo fix

[axrayn]

@colinsurprenant

Quick docs update to add:

• tables information into the index.asciidoc update docs with new table support #55
• example of how to specify the SNMPv3 settings - v3 user must have a "security_name" option, when security_name is already provided. #53
• fixed up a typo in the oid_root_skip definition in the table oid_root_skip noted as boolean instead of number #56

[colinsurprenant]

Thanks a lot @axrayn !
LGTM
@karenzone could you also take a look?

[karenzone]

@axrayn Builds cleanly and formatting looks great. Nice improvements to the plugin and the docs. Thanks for your contribution.