docs(readme): add usage + example

readme.md

@@ -1 +1,42 @@
 A simple template for French letters for typst.
+
+## Usage
+
+To create a letter, write a typst document while defining the following
+parameters, with the `lettre.typ` file in the same folder:
+
+```typst
+#import "lettre.typ": *
+#show link: strong
+
+#show: doc => lettre(
+  de: [
+    Sender\
+    Address
+  ],
+  pour: [
+    Recipient\
+    Address
+  ],
+  objet: "subject of the letter", // optional
+  date: "date of sending", // optional
+  lieu: "location",
+  introduction: "opening",
+  cloture: "closing",
+  signature: "signature",
+  post: [
+    post-letter (e.g., post-scriptum)
+  ],
+  doc
+)
+```
+
+Now you can fill the letter afterward, and compile your document with:
+
+```bash
+typst compile <filename.typ>
+```
+
+## Example
+
+An example file is given in the [`exemple.typ`](./exemple.typ) file in the repository.