Request to have a development guide about primihub

[phoenix20162016]

1. https://docs.primihub.com/docs/core-concept/model
   following this doc, you can reimplement the function which can meet your demands
2. to comiple primihub node, you just using make command and specifiy the platform what you need, such as:
   TARGET := //:node
   //:cli
   //src/primihub/pybind_warpper:linkcontext
   //src/primihub/pybind_warpper:opt_paillier_c2py
   //:py_main
   linux_aarch64:
   bazel build --config=linux_aarch64 ${TARGET}
3. if you find bug during using primihub service, you are highly welcome to create PR to help us fix it, what you just do is fork primihub repo, and then fix the problem based on develop branch, and then create a pull request to our develop, specifiy a reviewer, if the CICD is passed, we will merger your PR as soon as possible, and finally thanks for your contribution.