Added a note to api documentation for following APIs
1. PUT /vnfpkgm/v1/vnf_packages/{vnf_package_id}/package_content
2. POST /vnf_packages/{vnf_package_id}/package_content/upload_from_uri
The hashing algorithm of software images should be the same as
supported by glance.
Change-Id: I50dd3f8bc702e15df6a309a73e90a0c42fcd9d44