Document DWaveSampler use

[randomir]

Motivated by #138, #134 and #135:

Add an example of QBSolv use with EmbeddingComposite(DWaveSampler()) for subproblems solver. The example could go under the Introduction section and/or the QBSolv.sample() section.

[arcondello]

The EmbeddingComposite should not be used by QBSolv, the FixedEmbeddingComposite is preferred. See

qbsolv/examples/useFixedEmbeddingComposite.py

Line 1 in 1e38d78

import random

That example should be updated to use find_clique_embedding in dwave-system and it should be referenced more prominently.

[ioannist]

Trying to wrap my head around this example...You are using a complete graph for the subproblem so that it can represent any problem. Is that realistic or efficient in production? Is this the only way to split the problem?