Update adjoint documentation. (#4168)

* manual section on adjoints

Author: dham

docs/source/_static/references.bib

@@ -6,6 +6,19 @@ @comment{
 This bibtex file is for references in the documentation that aren't specifically to Firedrake.
 }
 
+@article{Farrell2013,
+  author    = {Farrell, Patrick E and Ham, David A and Funke, Simon W and Rognes, Marie E},
+  doi       = {10.1137/120873558},
+  journal   = {SIAM Journal on Scientific Computing},
+  number    = {4},
+  pages     = {C369--C393},
+  publisher = {SIAM},
+  title     = {Automated derivation of the adjoint of high-level transient finite element programs},
+  url       = {https://doi.org/10.1137/120873558},
+  volume    = {35},
+  year      = {2013}
+}
+
 @article{Gopalakrishnan2009,
   author    = {Jayadeep Gopalakrishnan and Shuguang Tan},
   doi       = {10.1002/nla.636},
@@ -19,3 +32,16 @@ @article{Gopalakrishnan2009
   volume    = {16},
   year      = {2009}
 }
+
+@article{Mitusch2019,
+  author    = {Mitusch, Sebastian and Funke, Simon and Dokken, J{\o}rgen},
+  doi       = {10.21105/joss.01292},
+  journal   = {Journal of Open Source Software},
+  number    = {38},
+  pages     = {1292},
+  publisher = {The Open Journal},
+  title     = {dolfin-adjoint 2018.1: automated adjoints for FEniCS and Firedrake},
+  url       = {https://doi.org/10.21105/joss.01292},
+  volume    = {4},
+  year      = {2019}
+}