add pre processing to list

Author: Mark Needham

doc/asciidoc/index.adoc

@@ -21,6 +21,7 @@ The guide covers the following areas:
 * <<algorithms-centrality>> -- A detailed guide to each of the centrality algorithms, including use-cases and examples.
 * <<algorithms-community>> -- A detailed guide to each of the community detection algorithms, including use-cases and examples.
 * <<algorithms-path-finding>> -- A detailed guide to each of the path finding algorithms, including use-cases and examples.
+* <<algorithms-preprocessing>> -- A detailed guide to each of the preprocessing functions and procedures.
 * <<algorithms-similarity>> -- A detailed guide to each of the similarity algorithms, including use-cases and examples.
 
 [[introduction]]