docs(readme): add Install, Stability and versioning sections for v1

Documents the v1.0.0 SemVer commitment and the deprecate-don't-delete
policy so consumers know what to expect from the v1 line. Adds an
explicit Install section with the minimum supported Go version
(1.26) and points migrators at the docs site for v1 release notes
and migration guide.

Linear: OSS-3393

Author: james00012

README.md

@@ -22,7 +22,24 @@ variety of helper functions and patterns for common infrastructure testing tasks
 - Running shell commands
 - And much more
 
-Please see the following for more info:
+## Install
+
+```bash
+go get github.com/gruntwork-io/terratest@latest
+```
+
+Requires Go 1.26 or later.
+
+## Stability and versioning
+
+Starting with v1.0.0, Terratest follows [semantic versioning](https://semver.org/). Breaking changes to the public API
+only happen in major releases (e.g. v2.0.0).
+
+Symbols renamed or replaced in v1 are kept with `// Deprecated:` annotations pointing at the new name; removals happen
+in v2. Migrating from v0.x: see the [v1 release notes and migration guide](https://terratest.gruntwork.io/docs/) on the
+Terratest docs site.
+
+## More info
 
 - [Terratest Website](https://terratest.gruntwork.io)
 - [Getting started with Terratest](https://terratest.gruntwork.io/docs/getting-started/quick-start/)