README: Fix broken URLs (Issue #138)

OVERVIEW.md

@@ -9,6 +9,10 @@ md <- md[-seq_len(grep("^## ", md)[1]-1)]
 ## Drop the footer
 md <- md[seq_len(grep("^---", md)[1]-1)]
 
+## Turn otherwise local links to CRAN for README.md
+md <- gsub(": (future-.*[.]html)",
+           ": https://cran.r-project.org/web/packages/future/vignettes/\\1", md)
+
 ## Output
 cat(md, sep="\n")
 %>

README.md

@@ -583,22 +583,15 @@ The goal of this package is to provide a standardized and unified API for using
 [future.batchjobs]: https://github.com/HenrikBengtsson/future.batchtools
 [globals]: https://cran.r-project.org/package=globals
 [listenv]: https://cran.r-project.org/package=listenv
-[Futures in R: Common Issues with Solutions]: future-2-issues.html
-[Futures in R: Future Topologies]: future-3-topologies.html
+[Futures in R: Common Issues with Solutions]: https://cran.r-project.org/web/packages/future/vignettes/future-2-issues.html
+[Futures in R: Future Topologies]: https://cran.r-project.org/web/packages/future/vignettes/future-3-topologies.html
 
 ## Installation
 R package future is available on [CRAN](https://cran.r-project.org/package=future) and can be installed in R as:
 ```r
 install.packages('future')
 ```
 
-### Pre-release version
-
-To install the pre-release version that is available in Git branch `develop` on GitHub, use:
-```r
-source('http://callr.org/install#HenrikBengtsson/future@develop')
-```
-This will install the package from source.