Clarify the requirements to build a cxx_library
Clarify the requirements to build a cxx_library

Summary: - Clarify that users must specify to Buck a top-level target--such as a `cxx_binary` or `android_binary`--in order for Buck to build a `cxx_library`.

Reviewed By: bobyangyf

fbshipit-source-id: eb0339936d

rmaz/buck,brettwooldridge/buck,brettwooldridge/buck,JoelMarcey/buck,SeleniumHQ/buck,JoelMarcey/buck,facebook/buck,SeleniumHQ/buck,rmaz/buck,rmaz/buck,Addepar/buck,nguyentruongtho/buck,kageiit/buck,zpao/buck,Addepar/buck,brettwooldridge/buck,romanoid/buck,nguyentruongtho/buck,brettwooldridge/buck,zpao/buck,romanoid/buck,rmaz/buck,facebook/buck,JoelMarcey/buck,JoelMarcey/buck,SeleniumHQ/buck,zpao/buck,kageiit/buck,Addepar/buck,nguyentruongtho/buck,romanoid/buck,kageiit/buck,JoelMarcey/buck,facebook/buck,kageiit/buck,brettwooldridge/buck,JoelMarcey/buck,SeleniumHQ/buck,brettwooldridge/buck,romanoid/buck,rmaz/buck,SeleniumHQ/buck,Addepar/buck,Addepar/buck,rmaz/buck,brettwooldridge/buck,JoelMarcey/buck,JoelMarcey/buck,kageiit/buck,SeleniumHQ/buck,Addepar/buck,zpao/buck,romanoid/buck,rmaz/buck,nguyentruongtho/buck,nguyentruongtho/buck,romanoid/buck,nguyentruongtho/buck,romanoid/buck,brettwooldridge/buck,rmaz/buck,Addepar/buck,JoelMarcey/buck,romanoid/buck,brettwooldridge/buck,romanoid/buck,zpao/buck,romanoid/buck,rmaz/buck,SeleniumHQ/buck,romanoid/buck,SeleniumHQ/buck,Addepar/buck,Addepar/buck,romanoid/buck,kageiit/buck,SeleniumHQ/buck,Addepar/buck,rmaz/buck,facebook/buck,SeleniumHQ/buck,SeleniumHQ/buck,zpao/buck,facebook/buck,brettwooldridge/buck,zpao/buck,kageiit/buck,romanoid/buck,rmaz/buck,JoelMarcey/buck,brettwooldridge/buck,Addepar/buck,facebook/buck,rmaz/buck,Addepar/buck,Addepar/buck,brettwooldridge/buck,nguyentruongtho/buck,brettwooldridge/buck,JoelMarcey/buck,SeleniumHQ/buck,facebook/buck,JoelMarcey/buck,SeleniumHQ/buck,JoelMarcey/buck,rmaz/buck