Added documentation

docs/primary_key.md

@@ -1,32 +1,45 @@
-# `sqlgen::PrimaryKey` 
+# `sqlgen::PrimaryKey`
 
-Sometimes you would like to set primary keys on your
-tables. This is possible as well:
+`sqlgen::PrimaryKey` is used to indicate which key should be a primary key.
+
+## Usage
+
+### Basic Definition
+
+Define a primary key field in your struct:
 
 ```cpp
 struct People {
-    rfl::PrimaryKey<std::string> first_name;
+    sqlgen::PrimaryKey<std::string> first_name;
     std::string last_name;
     uint age;
 };
 ```
 
-Now, the generated SQL code will look something like this:
+This generates the following SQL schema:
 
 ```sql
-CREATE TABLE IF NOT EXISTS "People"("first_name" TEXT NOT NULL, 
-                                    "last_name"  TEXT NOT NULL, 
-                                    "age"        INTEGER NOT NULL,
-                                    PRIMARY_KEY("first_name"));
+CREATE TABLE IF NOT EXISTS "People"(
+    "first_name" TEXT NOT NULL, 
+    "last_name"  TEXT NOT NULL, 
+    "age"        INTEGER NOT NULL,
+    PRIMARY_KEY("first_name")
+);
 ```
 
-`rfl::PrimaryKey<...>` is a simple wrapper. You can simply assign to it as follows:
+### Assignment and Access
+
+Assign values to primary key fields:
 
 ```cpp
-const auto person = People{.first_name = "Homer", ...};
+const auto person = People{
+    .first_name = "Homer",
+    .last_name = "Simpson",
+    .age = 45
+};
 ```
 
-And you can retrieve the underlying value using any of the following:
+Access the underlying value using any of these methods:
 
 ```cpp
 person.first_name();
@@ -34,3 +47,14 @@ person.first_name.get();
 person.first_name.value();
 ```
 
+## Notes
+
+- The template parameter specifies the type of the primary key field
+- Primary key fields are automatically marked as NOT NULL in the generated SQL
+- The class supports:
+  - Direct value assignment
+  - Multiple access methods for the underlying value
+  - Reflection for SQL operations
+  - Move and copy semantics
+- Primary keys can be used with any supported SQL data type
+