added README.md

.Rbuildignore

@@ -3,3 +3,4 @@ benchmark
 ^\.Rproj\.user$
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+README.md

README.md

+# Public repository for `dotCall64` package
+
+
+* CRAN package link: https://CRAN.R-project.org/package=dotCall64
+
+* `dotCall64` was by-product of the 64-bit extension of `spam` to `spam64`.  
+  For detailed examples, consult a recent source of the `spam` package. (E.g., https://cran.r-project.org/src/contrib/spam_2.8-0.tar.gz).   
+  The function `spam::bandwidth()` illustrates the R side of calling `.C64()`.
+
+* Yifan Liu has written a comprehensive and detailed tutorial, available at
+  https://fortran-lang.discourse.group/t/extend-r-with-fortran/2386.
+