add reference for hilbert_dist

Author: ytdHuang

docs/src/resources/bibliography.bib

@@ -81,3 +81,18 @@ @book{Wiseman2009Quantum
   year={2009},
   month=nov
 } 
+
+@article{Vedral-Plenio1998,
+  title = {Entanglement measures and purification procedures},
+  author = {Vedral, V. and Plenio, M. B.},
+  journal = {Phys. Rev. A},
+  volume = {57},
+  issue = {3},
+  pages = {1619--1633},
+  numpages = {0},
+  year = {1998},
+  month = {Mar},
+  publisher = {American Physical Society},
+  doi = {10.1103/PhysRevA.57.1619},
+  url = {https://link.aps.org/doi/10.1103/PhysRevA.57.1619}
+}

src/metrics.jl

@@ -47,6 +47,9 @@ Calculates the Hilbert-Schmidt distance between two [`QuantumObject`](@ref):
 ``D_{HS}(\hat{\rho}, \hat{\sigma}) = \textrm{Tr}\left[\hat{A}^\dagger \hat{A}\right]``, where ``\hat{A} = \hat{\rho} - \hat{\sigma}``.
 
 Note that `ρ` and `σ` must be either [`Ket`](@ref) or [`Operator`](@ref).
+
+# References
+- [Vedral-Plenio1998](@citet)
 """
 function hilbert_dist(
     ρ::QuantumObject{ObjType1},