Add config to README

[dima-shulga]

In install process you have to wtite that it's needed to add the mixins property to server/model-config.json:

"_meta": {
"sources": [
"loopback/common/models",
"loopback/server/models",
"../common/models",
"./models"
],
"mixins": [
"loopback/common/mixins",
"loopback/server/mixins",
"../node_modules/loopback-softdelete-mixin",
"../common/mixins",
"./mixins"
]
},