merge.go: short description

Sereal's merger is capabale of merging two Sereal documents
into a single one without deserialization/serialization
cycle.

Available options:

TopLevelElement
TopLevelElement allows a user to choose what container will be used at top level.

DedupeStrings
If enabled, merger will deduplicate all strings it meets.
Otherwise, only hash key and class names will be deduplicated

KeepFlat bool
If enabled, KeepFlat keeps flat structture of final document.
Specifically, consider two arrays [A,B,C] and [D,E,F]:
- when KeepFlat == false, the result of merging is [[A,B,C],[D,E,F]]
- when KeepFlat == true, the result is [A,B,C,D,E,F]
  This mode is relevant only to top level elements

The work is not done yet, but the tool is good enough to
be available to wider audience.

TODO:
* write tests
* add header support
* memory optimizations