Add driver plugins readme

diff --git a/plugins/drivers/README.md b/plugins/drivers/README.md
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+Developing drivers: https://www.adminer.org/en/drivers/
+
+The type declarations must be compatible both with source codes and the compiled version (where PHP5-incompatible types are stripped). It means:
+- specify return type if parent specifies it
+- do not specify scalar parameter type