Updating the documentation to tell people about v3

.travis.yml

@@ -5,6 +5,7 @@ go:
   - 1.10.x
   - 1.11.x
   - 1.12.x
+  - 1.13.x
   - tip
 
 # Setting sudo access to false will let Travis CI use containers rather than

README.md

@@ -1,4 +1,5 @@
 # Sprig: Template functions for Go templates
+
 [![Stability: Sustained](https://masterminds.github.io/stability/sustained.svg)](https://masterminds.github.io/stability/sustained.html)
 [![Build Status](https://travis-ci.org/Masterminds/sprig.svg?branch=master)](https://travis-ci.org/Masterminds/sprig)
 
@@ -11,6 +12,18 @@ It is inspired by the template functions found in
 [Twig](http://twig.sensiolabs.org/documentation) and in various
 JavaScript libraries, such as [underscore.js](http://underscorejs.org/).
 
+## Package Versions
+
+There are two active major versions of the `sprig` package.
+
+* v3 is currently under development on the `master` branch. The Go API should
+  remain compatible with v2, the current stable version. Behavior change behind
+  some functions is the reason for the new major version.
+* v2 is the current stable release series. It has been more than three years since
+  the initial release of v2. You can read the documentation and see the code
+  on the [release-2](https://github.com/Masterminds/sprig/tree/release-2) branch.
+  Bug fixes to this major version will continue for some time.
+
 ## Usage
 
 **Template developers**: Please use Sprig's [function documentation](http://masterminds.github.io/sprig/) for