Add Ammann '78 paper.

This is probably the most terse paper I have ever read. But it seems to be the
first to use the idea of follow sets (though it took me several reads to guess
that's the idea being presented).

Author: ltratt

softdev.bib

@@ -2164,3 +2164,13 @@ @book{holub90compilerdesign
     year = 1990,
     publisher = "Prentice-Hall"
 }
+
+@techreport{ammann78error,
+    author = "Urs Ammann",
+    title = "Error recovery in recursive descent parsers",
+    number = "136031",
+    institution = "ETH Zürich",
+    year = "1978",
+    month = May,
+    doi = "10.3929/ethz-a-000136031"
+}