Added doc reference for invdigamma, trigamma , polygamma(#25) (#134)

Author: Sumegh-git

docs/src/index.md

@@ -16,6 +16,9 @@ libraries.
 | [`sinint(x)`](@ref SpecialFunctions.sinint)                   | [sine integral](https://en.wikipedia.org/wiki/Trigonometric_integral) at `x` |
 | [`cosint(x)`](@ref SpecialFunctions.cosint)                   | [cosine integral](https://en.wikipedia.org/wiki/Trigonometric_integral) at `x` |
 | [`digamma(x)`](@ref SpecialFunctions.digamma)                 | [digamma function](https://en.wikipedia.org/wiki/Digamma_function) (i.e. the derivative of `lgamma` at `x`)                                                     |
+| [`invdigamma(x)`](@ref SpecialFunctions.invdigamma)   | [invdigamma function](http://bariskurt.com/calculating-the-inverse-of-digamma-function/) (i.e. inverse of `digamma` function at `x` using fixed-point iteration algorithm) |
+| [`trigamma(x)`](@ref SpecialFunctions.trigamma)     | [trigamma function](https://en.wikipedia.org/wiki/Trigamma_function) (i.e the logarithmic second derivative of `gamma` at `x`) |
+| [`polygamma(m,x)`](@ref SpecialFunctions.polygamma)  | [polygamma function](https://en.wikipedia.org/wiki/Polygamma_function) (i.e the (m+1)-th derivative of the `lgamma` function at `x`) |
 | [`eta(x)`](@ref SpecialFunctions.eta)                         | [Dirichlet eta function](https://en.wikipedia.org/wiki/Dirichlet_eta_function) at `x`                                                                           |
 | [`zeta(x)`](@ref SpecialFunctions.zeta)                       | [Riemann zeta function](https://en.wikipedia.org/wiki/Riemann_zeta_function) at `x`                                                                             |
 | [`airyai(z)`](@ref SpecialFunctions.airyai)                   | [Airy Ai function](https://en.wikipedia.org/wiki/Airy_function) at `z`                                                                                          |