Add note in README that these docs are for v3.

Author: sirbrillig

README.md

@@ -4,6 +4,8 @@
 
 Plugin for PHP_CodeSniffer static analysis tool that adds analysis of problematic variable use.
 
+Please note that this README is for VariableAnalysis v3. For documentation about v2, [see this page](https://github.com/sirbrillig/phpcs-variable-analysis/blob/2.8.2/README.md).
+
 - Warns if variables are used without being defined. (Sniff code: `VariableAnalysis.CodeAnalysis.VariableAnalysis.UndefinedVariable`)
 - Warns if variables are set or declared but never used. (Sniff code: `VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable`)
 - Warns if function parameters are declared but never used. (Sniff code: `VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedParameter`)