diff --git a/toolchains/posix/README.md b/toolchains/posix/README.md
index 28a3c0e0..d055a761 100644
--- a/toolchains/posix/README.md
+++ b/toolchains/posix/README.md
@@ -16,7 +16,7 @@ Rules for importing a POSIX toolchain from Nixpkgs.
 ### nixpkgs_sh_posix_configure
 
 <pre>
-nixpkgs_sh_posix_configure(<a href="#nixpkgs_sh_posix_configure-name">name</a>, <a href="#nixpkgs_sh_posix_configure-packages">packages</a>, <a href="#nixpkgs_sh_posix_configure-kwargs">kwargs</a>)
+nixpkgs_sh_posix_configure(<a href="#nixpkgs_sh_posix_configure-name">name</a>, <a href="#nixpkgs_sh_posix_configure-packages">packages</a>, <a href="#nixpkgs_sh_posix_configure-exec_constraints">exec_constraints</a>, <a href="#nixpkgs_sh_posix_configure-kwargs">kwargs</a>)
 </pre>
 
 Create a POSIX toolchain from nixpkgs.
@@ -62,6 +62,20 @@ default is <code>["stdenv.initialPath"]</code>
 
 List of Nix attribute paths to draw Unix tools from.
 
+</p>
+</td>
+</tr>
+<tr id="nixpkgs_sh_posix_configure-exec_constraints">
+<td><code>exec_constraints</code></td>
+<td>
+
+optional.
+default is <code>None</code>
+
+<p>
+
+Constraints for the execution platform.
+
 </p>
 </td>
 </tr>