Added ability to specify compatibility in hotfix

docs/EXPRESSIONS.md

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+Matching Expressions
+====================
+
+Hotfixes can provide a `for` key with an array of supported systems. The
+expressions are evaluated using Symfony's ExpressionLanguage and as such
+can handle some pretty complex stuff.
+
+For a hotfix to be considered supported, at least one of the expressions
+must evaluate to true.
+
+
+## Example:
+
+    for:
+        - lsb.id=='raspbian'
+        - lsb.id=='ubuntu' and lsb.release=>'16.04'
+
+
+## System facts
+
+### Distribution
+
+Information is retrieved using `lsb_release` or the `/etc/os-release` file.
+
+      lsb.id            Distribution ID (ex. ubuntu or raspbian)
+      lsb.description   Pretty description
+      lsb.release       Version number (ex. 16.04 or 8)
+      lsb.codename      Release codename (ex. xenial, not always present)
+