docs: init the project readme

README.md

@@ -1,3 +1,56 @@
 # howmuch
 
-A tricount like expense-sharing system written in Go
+A tricount like expense-sharing system written in Go
+
+---
+
+It is a personal project to learn go and relative technologies.
+
+---
+
+## Project Diary
+
+### 2024/09/30
+
+The idea comes from a discussion with my mom. I was thinking about doing
+some personal budget management thing but she brought up the expense-sharing
+application that could be a good idea. I explained why it was a terrible idea
+and had no value but in fact it was a really a good idea.
+
+First I have to set up a web server. I'm thinking about using `gin`, since I
+have played with `chi` in other projects.
+
+Then I have to add some basic support functions like system `logging`,
+versioning, and other stuffs.
+
+Next I need to design the API.
+
+- User management: signup, login, logout.
+- A logged-in user must be able to:
+  - create an event
+  - add other users to that event
+  - A user can only view their own events, but not the events of other users'
+  - A user can add an expense to the event (reason, date, who payed how much,
+  who benefited how much)
+  - Users in the event can edit or delete one entry
+  - changes are sent to friends in the event
+  - User can get the money they spent themselves and the money they must pay
+  to each other
+  - User can also get the total amount or the histories.
+
+That is what I thought of for now.
+
+Thus, Besides a web server, I must have a database that can store all the
+data. ex. PostgreSQL. I need a message queue system (RabbitMQ?) to handle
+changes for an event. That will results in a messaging service sending emails.
+
+I also want to use `Redis` for cache management.
+
+What else?
+
+`OpenAPI` + `swagger` for API management.
+
+And last but not least, `Docker` + `Kubernetes` for the deployment.
+
+That is what I am thinking of for now. I will note down other ideas during
+the project.