Add code_structure & database_design doc

Signed-off-by: Arnob kumar saha <arnob@appscode.com>

docs/architecture.md

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+Description on each of the packages
+
+# cmd
+Under the root command, there is only one subcommand named start_server. We use the routers package from it & start the server.
+
+# routes
+We have these types of routes: 
+
+`user_auth`: signUp, signIn, signOut
+
+`course`: 
+i) get/list -> general authenticated users can do it.
+ii) create/update -> admin or moderators can do.
+iii) delete -> only admin can do it.
+
+For get,update & delete calls(those work on a specific course uid), we utilize a context middleware for injecting context data.
+
+There is one special route for providing role to a user. It requires the admin access.
+
+# pkg
+-`pkg.auth`:
+
+We store these fields in the redis session:
+i) authenticated, ii) role, iii) userName, iv) userIP, v) user_agent
+There are some getters implemented in the session.go file.
+
+
+
+-`pkg.error`
+
+-`pkg.middleware`:
+
+There are 4 types of middlewares in this package.
+i) global middleware: For example logger, urlFormatter etc.
+
+ii) common security middleware: To check if a session is valid & authenticated.
+
+iii) access related middleware: To check if the role(in session) is matched.
+
+iv) context middleware: To append additional info to the context.
+
+
+# handlers
+routes -> handlers -> models
+
+# models
+1) `models.course`
+2) `models.db`
+3) `models.user`
+
+

docs/database_design.md

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+## API
+```
+type Course struct {
+CourseId           string    `json:"courseId" bson:"_id"`
+Title              string    `json:"title" bson:"title"`
+Description        string    `json:"description" bson:"description"`
+Instructors        []string  `json:"instructors" bson:"instructors"`
+Moderators         []string  `json:"moderators" bson:"moderators"`
+StartDate          date.Date `json:"startDate" bson:"startDate"`
+EndDate            date.Date `json:"endDate" bson:"endDate"`
+Duration           int       `json:"duration" bson:"duration"` // Duration in week
+Capacity           int       `json:"capacity" bson:"capacity"`
+Students           []string  `json:"students" bson:"students"`
+Price              string    `json:"price" bson:"price"`
+}
+
+
+
+type Lesson struct {
+UUID     string   `json:"uuid" bson:"uuid"`
+Serial   string   `json:"uuid" bson:"serial"`
+Title    string   `json:"title" bson:"title"`
+Content  []string `json:"content" bson:"content"`
+}
+
+type Content struct {
+ID string `json:"uuid" bson:"_id"`
+Name string `json:"Name" bson:"name"`
+Type string `json:"type" bson:"type"` // video, resource, quiz, lab
+}
+```
+
+
+## A complete Example
+```
+Course name: Mastering golang
+Lessons:
+Introduction:
+    Why Go?
+    Installation
+    <resources>
+    <quiz>
+Concurrency in Go:
+    channels
+    mutex
+    <lab1>
+    <resources>
+    go routines
+    <lab2>
+Conclusion
+    thank you
+```
+
+For the lab1 under 'Concurrency in Go', url will be like:
+http://praromvik.com/course/mastering_golang/concurrency_lab_1/
+Intentionally cutting off the lesson id part to keep the url shorter.
+
+So the restrictions are:
+i) course ids have to be unique. 
+ii) content ids within a course have to be unique.
+iii) both course & content ids have to be <= 24 characters, as they will be used as _id in mongodb.
+
+
+### `praromvik.courses` collection
+```
+{
+    "_id": "mastering_golang",
+    "title": "Mastering Golang",
+    "instructors": []string{<some_user_ids>},
+    "students": []string{<some_user_ids>}
+}
+```
+### `mastering_golang.lessons` collection
+```
+{
+    "_id": "<mongo-generated-id>",
+    "title": "Introduction",
+    "serial": 1,
+    "contents": []string{<_ids>}
+},
+{
+    "_id": "<mongo-generated-id>",
+    "title": "Concurrency in Go",
+    "serial": 2,
+    "contents": []string{"channel", "concurrency_lab_1", "resouce_from_doc"}
+}
+```
+
+### `mastering_golang.contents` collection
+```
+{
+    "_id": "channel",
+    "name": "Channel",
+    "type": "Video"
+},
+{
+    "_id": "concurrency_lab_1",
+    "name": "Hands-on the Go channel",
+    "type": "Lab"
+},
+{
+    "_id": "resouce_from_doc",
+    "name": "Additional learing resouces",
+    "type": "Written"
+},
+```