Add introductory documentation

[fgm]

From discussions with prospective users, it appears that the very notion of a DIC, familiar though it may be for Java (Spring DI / Beans, Dagger) or PHP (Symfony, PSR/11), may not be obvious for Go programmers, for whom the "container" word tends to evoke deployment technologies more than code organization tools.

The doc should introduce the concepts, with pointers to IoC, DI patterns, and explanations about the issues arising when using a DIC in the Service Locator pattern.