Add "String fields and column length" sections to docs

docs/tutorial/str-fields-and-column-length.md

@@ -0,0 +1,56 @@
+# String fields and column length
+
+Some databases have a limit on the length of string columns (e.g., `VARCHAR(255)` in *MySQL*) and fail with an error if you try to create a string column without specifying a length.
+
+**SQLModel** handles this automatically depending on the database dialect you are using. 😎
+
+For databases that require a length for string columns, **SQLModel** will automatically set a default length (e.g., `255` for *MySQL*) if you do not specify one.
+
+{* ./docs_src/tutorial/str_fields_and_column_length/tutorial001_py310.py ln[4:6] hl[6] *}
+
+If you run this code with *MySQL*, **SQLModel** will create the `name` column as `VARCHAR(255)`:
+
+```sql
+CREATE TABLE hero (
+        id INTEGER NOT NULL AUTO_INCREMENT,
+        name VARCHAR(255) NOT NULL,
+        PRIMARY KEY (id)
+)
+```
+
+But you can always specify a custom length if needed:
+
+{* ./docs_src/tutorial/str_fields_and_column_length/tutorial002_py310.py ln[4:6] hl[6] *}
+
+```sql
+CREATE TABLE hero (
+        id INTEGER NOT NULL AUTO_INCREMENT,
+        name VARCHAR(100) NOT NULL,
+        PRIMARY KEY (id)
+)
+```
+This works thanks to `AutoString` type that **SQLModel** uses for all string fields by default.
+
+But if you specify the database type of column explicitly, **SQLModel** will not be able to set the length automatically, and you will need to specify it manually:
+
+{* ./docs_src/tutorial/str_fields_and_column_length/tutorial003_py310.py ln[1:6] hl[1,6] *}
+
+The code example above will fail on databases that require a length for string columns:
+
+```console
+sqlalchemy.exc.CompileError: (in table 'hero', column 'name'): VARCHAR requires a length on dialect mysql
+```
+
+To fix it, you need to specify the length explicitly as follows:
+
+{* ./docs_src/tutorial/str_fields_and_column_length/tutorial004_py310.py ln[1:6] hl[1,6] *}
+
+This will give:
+
+```sql
+CREATE TABLE hero (
+        id INTEGER NOT NULL AUTO_INCREMENT,
+        name VARCHAR(255) NOT NULL,
+        PRIMARY KEY (id)
+)
+```