Document how to treat ordering of members in BOM

Document how to treat ordering of members in BOM. Users may pick up arbitrary order of BOM members.

Assume there's A, B and C in BOM where
- Class path `A.jar:B.jar:C.jar` generates zero static linkage errors
- Class path `B.jar:A.jar:C.jar` generates 1 static linkage error

What should a static linkage check say about the situation?

# Option 0
Advice BOM users to follow an order.

# Option 1
Run static linkage check on all permutation. Too expensive.

# Option 2
Given a reference, check all available jar files that contains the referent class. For each version of jar, check compatibility
This will generate false positives.

# Option 3
Round-robin table (pair-wise comparison) #168. Something similar to
[Cloud Opensource Python: Compatibility Grid](https://googlecloudplatform.github.io/cloud-opensource-python/grid.html)

Note that when we do pair-wise comparison, the dependency tree of each cell needs to use managed dependency as if it's fetching them via BOM.