Add documentation for Signature.new

Author: lizmat

doc/Type/Signature.pod6

@@ -1048,6 +1048,35 @@ Defined as:
 
 Throws C<X::Cannot::Capture>.
 
+=head1 Runtime creation of Signature objects (6.d, 2019.01 and later)
+
+    Signature.new(params => (...), returns => Type, arity => 1, count => 1)
+
+In some situations, specifically when working with the MetaObject Protocol,
+it makes sense to create C<Signature> objects programmatically.  For this
+purpose, you can call the C<new> method with the following named parameters:
+
+=item params
+
+A list of L<Parameter|/type/Parameter> objects for this signature.
+
+=item returns
+
+Any constraint the return value should match.  Defaults to C<Mu>, which
+effectively implies no return value constraint check.
+
+=item arity
+
+The I<minimal> number of positional arguments required to satisfy the
+signature.  Defaults to the number of C<Parameter> objects given with
+the C<params> parameter.
+
+=item count
+
+The I<maximal> number of positional arguments which can be bound to the
+signature. Defaults to the C<arity> if not specified.  Specify C<Inf> if
+there is a slurpy positional parameter.
+
 =end pod
 
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