add docs for the OID_BASIC_CONSTRAINTS constant
diff --git a/docs/x509.rst b/docs/x509.rst
index 89265df..18f6ff1 100644
--- a/docs/x509.rst
+++ b/docs/x509.rst
@@ -473,6 +473,14 @@
     Corresponds to the dotted string ``2.16.840.1.101.3.4.3.2"``. This is
     a SHA256 digest signed by a DSA key.
 
+Extension OIDs
+~~~~~~~~~~~~~~
+
+.. data:: OID_BASIC_CONSTRAINTS
+
+    Corresponds to the dotted string ``"2.5.29.19"``. The identifier for the
+    basic constraints extension.
+
 
 Exceptions
 ~~~~~~~~~~