feat(readme): add readme file

README.md

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+# LU bookstore
+
+<!-- toc -->
+
+- [Task](#task)
+- [Requirements](#requirements)
+  * [Data structure](#data-structure)
+  * [Functionality](#functionality)
+    + [Add book](#add-book)
+    + [Search by ISBN](#search-by-isbn)
+    + [Search by title or author](#search-by-title-or-author)
+    + [Book list](#book-list)
+    + [Delete book](#delete-book)
+  * [User interface](#user-interface)
+  * [Submission](#submission)
+
+<!-- tocstop -->
+
+## Task
+Develop a bookstore inventory management system in Python.
+
+## Requirements
+
+### Data structure
+- [ ] The inventory list is stored as a Python dictionary.
+- [ ] The original list of books can be *hard coded* into the program to avoid having to re-enter the data each time.
+- [ ] At least the following information is stored for each book: `title`, `author`, `ISBN`, `price` and `quantity in stock`.
+- [ ] Information for each book is stored in a separate dictionary
+- [ ] The keys of the inventory dictionary are the `ISBN` codes and its values are the dictionaries containing the information about the book.
+
+### Functionality
+#### Add book
+- [ ] The user should be given the possibility to add a new book to the inventory.
+- [ ] When adding a book, make sure that its ISBN number is unique. If such an ISBN number already exists in the list, display an error message.
+#### Search by ISBN
+- [ ] Users should be allowed to search for a book by its ISBN number.
+- [ ] If the book was found, display information about it.
+- [ ] If the book was not found, display an error message.
+#### Search by title or author
+- [ ] Allow users to search for a book by name in its title or author field. The result is a list of books that match the search criteria.
+#### Book list
+- [ ] Display a list of all books, showing the following information for each book: `title`, `author`, `ISBN`, `quantity in stock`.
+#### Delete book
+- [ ] Delete a book with a given ISBN number from the list.
+- [ ] To inform the user that the book was successfully deleted or that the ISBN number was not found.
+
+### User interface
+
+For a score of 8, it is sufficient to implement a simple textual interface.
+
+For a higher score, develop a program with a graphical user interface (using Python libraries such as `Tkinter`, `PyQt` or `PySide`).
+You can also use the `rich` library, which allows you to create an *advanced* text format interface.
+
+### Submission
+Submit an archive with the source code of a Python program that implements the requested functionality.
+The program code must be properly documented (with comments and meaningful variable names).
+
+If external libraries are used, include in the archive a `requirements.txt` file containing the names and version numbers of these libraries.