C#: Add change note

Author: hvitved

change-notes/1.20/analysis-csharp.md

@@ -11,6 +11,8 @@
 
 | *@name of query (Query ID)*| *Impact on results*    | *How/why the query has changed*                                  |
 | Off-by-one comparison against container length (cs/index-out-of-bounds) | Fewer false positives | Results have been removed when there are additional guards on the index. |
+| Dereferenced variable is always null (cs/dereferenced-value-is-always-null) | Improved results | The query has been rewritten from scratch, and the analysis is now based on static single assignment (SSA) forms. The query is now enabled by default in LGTM. |
+| Dereferenced variable may be null (cs/dereferenced-value-may-be-null) | Improved results | The query has been rewritten from scratch, and the analysis is now based on static single assignment (SSA) forms. The query is now enabled by default in LGTM. |
 
 ## Changes to code extraction