feat: add README

README.md

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+# OnceCell
+
+A thread-safe value that can only be set once
+
+---
+
+Have you ever wanted to use a constant in your program, but did not have access to its value until runtime?
+
+Have you ever wanted to use a global variable to store command line arguments, but wanted to ensure the variable is never changed?
+
+Have you every wanted to enforce immutability with data shared by multiple threads?
+
+Well, this is the library for you!
+
+
+## Installation
+
+```bash
+go get github.com/Xavientois/once-cell/primitives
+```
+
+## Usage
+
+```go
+import oncecell "github.com/Xavientois/once-cell/primitives"
+
+var myString primitives.OnceString
+
+// Setters
+myString.SetValue("hello") // Set the value once
+err := myString.SetValue("world") // Set the value again and an error will be returned
+myString.MustSetValue("world") // Will panic if the value is already set
+
+// Getters
+s, err := myString.Value() // Returns an error if the value has not been set
+s := myString.MustValue() // Panics if the value has not been set
+
+```
+
+## Custom Types
+
+The library is currently only implemented for primitive types, at least until generics are introduced into the language.
+If you wish to use custom types in the meantime, you can use the `primitives/once-cell-gen.sh` script to generate an implementation for your custom type.
+
+```bash
+./primitives/once-cell-gen.sh YourTypeName
+```
+
+The second command-line argument is the value to be used in the generated tests
+
+## License
+
+Apache 2.0
+
+
+*Inspired by [once_cell](https://doc.rust-lang.org/std/lazy/struct.OnceCell.html) from Rust*