Update README about merge algorithms

[vargenau]

We currently support two algorithm

• Hierarchical: This merge algo tries to maintain, the order of the dependent components to its primary component. For spdx this is done via relationships and for cyclonedx via nested components & dependencies.
• Flat: As the name states, are just consolidated lists of components, dependencies, etc.
• Assembly: Merge is very similar to Hierarchical, except that it does not create dependency relationships among the merged sboms.

This is three algorithms, not two.

You should add that Flat is not yet implemented.

What is the default algorithm, i.e. when -a, -fand -mare not used?

[riteshnoronha]

The default algorithm is Hierarchal Merge for both SPDX and CycloneDX. Yes i plan on implementing both FLAT & Assembly merge for SPDX soon. Hopefully next week.

[riteshnoronha]

Added more details here #83