Add maxReaders option

This allows the size of LMDB's readers table to be set, making space for
more concurrent readers; necessary for lots of concurrent iterators.
Test maxReaders

Confirm that setting the `maxReaders` does indeed make more reader slots
available than not setting it.
Document maxReaders

Added a section with structure mirroring the others: Explanation of what
it is, how much it costs to set it higher and what happens if you don't.

Resolves #6.
Merge pull request #7 from c39/set-maxreaders

Allow maxreaders parameter to be set