updated CONTRIBUTING.md with guides to filing issues and contributing code
small modifications from PR feedbacl
Merge pull request #843 from realm/jp-contributing

updated CONTRIBUTING.md with guides to filing issues and contributing code
Document RLMRealm lifetimes and related issues

Should cover all of the points of confusion related to RLMRealm
lifetimes that people have run in to.
Merge pull request #844 from realm/tg-rlmrealm-docs

Document RLMRealm lifetimes and related issues
Use std::unique_ptr to hold pointers to C++ types

Cuts down on cleanup code and fixes a memory leak when creating a shared
group throws an exception.
Implement read-only realms