Document how to shrink the size of installed NPM

With some frequency, users have encountered large size installs of the couchbase NPM. The actual code is smaller and there is a way to shrink it down to size for most environments easily.

This should be documented with some priority since it's coming up regularly.

See also this forums post:
https://forums.couchbase.com/t/couchbase-nodejs-4-1-0-sdk-unzipped-size-causing-an-issue-for-lambda-layers/34766/2?u=ingenthr