Docs: comparison with mysql++, libmysqlclient, sqlpp11

[anarthal]

Add a comment to state the differences with these two libraries.

From the Boost review:

• Add a performance page in the documentation comparing the library with these two.
• Add a comparison to other C asynchronous APIs, in terms of ease of use.
• Clarify that the network protocol we're using here is stable.

[SplitGemini]

+1

[jwakely]

There's also http://mysqlcppapi.sourceforge.net/ but it's moribund (in theory I maintain it but I haven't touched it in years and recommend MySQL++ instead).