doc: document true command

Create a man-page for the true command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 1304b59..d3abf2d 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -24,3 +24,4 @@
    mbr
    pstore
    sbi
+   true