update readme

README.md

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-Todo
+## What is it?
+A pet project where I experiment/learn various libraries/technics/patterns.
+
+## Tech Stack
+  - Http4s
+  - Doobie
+  - Circe
+  - Cats
+  - Refined 
+  - NewType
+  - Tapir
+  - OpenTracing
+  - Enumeratum
+  - PureConfig
+  - Quill
+  - ScalaTest
+
+## Domain
+
+Car adverts should have the following fields:
+* **id** (_required_): **int** or **guid**, choose whatever is more
+  convenient for you;
+* **title** (_required_): **string**, e.g. _"Audi A4 Avant"_;
+* **fuel** (_required_): gasoline or diesel, use some type which
+  could be extended in the future by adding additional fuel types;
+* **price** (_required_): **integer**;
+* **new** (_required_): **boolean**, indicates if car is new or
+  used;
+* **mileage** (_only for used cars_): **integer**;
+* **first registration** (_only for used cars_): **date** without
+  time.
+
+## Functionality:
+
+* have functionality to return list of all car adverts;
+* optional sorting by any field specified by query parameter,
+  default sorting - by **id**;
+* have functionality to return data for single car advert by id;
+* have functionality to add car advert;
+* have functionality to modify car advert by id;
+* have functionality to delete car advert by id;
+* have validation (see required fields and fields only for used
+  cars);
+* accept and return data in JSON format, use standard JSON date
+  format for the **first registration** field.
+
+
+## Code
+
+#### Domain models 
+Case classes designed using NewType and Refined. I believe this approach provides the best experience when it comes to dealing with invalid data.
+
+#### API design: 
+I used Tapir to describe endpoints. I liked the idea of describing endpoints in type safe manner. At the beginning, there was a learning curve, but I really liked the final result.  
+
+#### Tagless Final:
+I was not really experience in that regard, but I wanted to use Tagless Final just to see how it feels. However, I think it was a bit overkill for this sample project.
+
+#### Validation:
+I used cats.Validated for validating user inputs and business logics. It's very easy to use, and at the end we get accumulated errors.
+
+#### Compiler Plugins: 
+I am using 4 compiler plugin in this project to enable some fancy(missing) features of scala compiler. I would suggest you to read github page of each plugin to at least understand why it exists.
+I think almost all of them won't be needed anymore once we have Scala 3