Added spec for special functions

Author: hsnyder

doc/specs/stdlib_specialfunctions.md

@@ -0,0 +1,64 @@
+---
+title: specialfunctions
+---
+
+# Special functions
+
+[TOC]
+
+## `legendre` - Calculate Legendre polynomials
+
+### Status
+
+Experimental
+
+### Description
+
+Computes the value of the n-th Legendre polynomial at a specified point.
+Currently only 64 bit floating point is supported.
+
+This is an `elemental` function.
+
+### Syntax
+
+`result = [[stdlib_specialfunctions(module):legendre(interface)]](n, x)`
+
+### Arguments
+
+`n`: Shall be a scalar of type `real(real64)`. 
+
+`x`: Shall be a scalar or array (this function is elemental) of type `real(real64)`. 
+
+### Return value
+
+The function result will be the value of the `n`-th Legendre polynomial, evaluated at `x`.
+
+
+
+## `dlegendre` - Calculate first derivatives of Legendre polynomials
+
+### Status
+
+Experimental
+
+### Description
+
+Computes the value of the first derivative of the n-th Legendre polynomial at a specified point.
+Currently only 64 bit floating point is supported.
+
+This is an `elemental` function.
+
+### Syntax
+
+`result = [[stdlib_specialfunctions(module):dlegendre(interface)]](n, x)`
+
+### Arguments
+
+`n`: Shall be a scalar of type `real(real64)`. 
+
+`x`: Shall be a scalar or array (this function is elemental) of type `real(real64)`. 
+
+### Return value
+
+The function result will be the value of the first derivative of the `n`-th Legendre polynomial, evaluated at `x`.
+