Replace external R style guide

Internally, we've moved to the Tidyverse Style Guide: style.tidyverse.org
The new file for Rguide.md reflects this fact and notes where we differ.

Rguide.md

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+# Google's R Style Guide
+
+R is a high-level programming language used primarily for statistical computing
+and graphics. The goal of the R Programming Style Guide is to make our R code
+easier to read, share, and verify.
+
+The Google R Style Guide is a fork of the
+[Tidyverse Style Guide](https://style.tidyverse.org/) by Hadley Wickham
+[license](https://creativecommons.org/licenses/by-sa/2.0/). Google modifications
+were developed in collaboration with the internal R user community. The rest of
+this document explains Google's primary differences with the Tidyverse guide,
+and why these differences exist.
+
+## Syntax
+
+### Naming conventions
+
+Google prefers identifying functions with `BigCamelCase` to clearly distinguish
+them from other objects.
+
+```
+# Good
+DoNothing <- function() {
+  return(invisible(NULL))
+}
+```
+
+The names of private functions should begin with a dot. This helps communicate
+both the origin of the function and its intended use.
+
+```
+# Good
+.DoNothingPrivately <- function() {
+  return(invisible(NULL))
+}
+```
+
+We previously recommended naming objects with `dot.case`. We're moving away from
+that, as it creates confusion with S3 methods.
+
+### Don't use attach()
+
+The possibilities for creating errors when using `attach()` are numerous.
+
+## Pipes
+
+### Right-hand assignment
+
+We do not support using right-hand assignment.
+
+```
+# Bad
+iris %>%
+  dplyr::summarize(max_petal = max(Petal.Width)) -> results
+```
+
+This convention differs substantially from practices in other languages and
+makes it harder to see in code where an object is defined. E.g. searching for
+`foo <-` is easier than searching for `foo <-` and `-> foo` (possibly split over
+lines).
+
+### Use explicit returns
+
+Do not rely on R's implicit return feature. It is better to be clear about your
+intent to `return()` an object.
+
+```
+# Good
+AddValues <- function(x, y) {
+  return(x + y)
+}
+
+# Bad
+AddValues <- function(x, y) {
+  x + y
+}
+```
+
+### Qualifying namespaces
+
+Users should explicitly qualify namespaces for all external functions.
+
+```
+# Good
+purrr::map()
+```
+
+We discourage using the `@import` Roxygen tag to bring in all functions into a
+NAMESPACE. Google has a very big R codebase, and importing all functions creates
+too much risk for name collisions.
+
+While there is a small performance penalty for using `::`, it makes it easier to
+understand dependencies in your code. There are some exceptions to this rule.
+
+*   Infix functions (`%name%`) always need to be imported.
+*   Certain `rlang` pronouns, notably `.data`, need to be imported.
+*   Functions from default R packages, including `datasets`, `utils`,
+   `grDevices`, `graphics`, `stats` and `methods`. If needed, you can `@import`
+   the full package.
+
+When importing functions, place the `@importFrom` tag in the Roxygen header
+above the function where the external dependency is used.
+
+## Documentation
+
+### Package-level documentation
+
+All packages should have a package documentation file, in a
+`packagename-package.R` file.