Add usage example

README.md

@@ -3,4 +3,49 @@ An action to replace variables in a given string.
 
 It expects a string containing any number of placeholders, and then attempts to populate them from the currently available environment variables.
 
-TODO: Add example usage.
+## Inputs
+
+### instring
+
+The input string containing the lowercase variable names, surrounded by curly braces. **required**
+
+### prefix
+
+The prefix to use when inspecting the environment. Only environment variables starting with the prefix will be used when replacing the placeholders. **optional**
+
+## Outputs
+
+### outstring
+
+The string after substituting the placeholders.
+
+## Example Usage
+
+```yaml
+name: String Replace Test
+on:
+  push:
+    branches: master
+jobs:
+  example:
+    runs-on: ubuntu-latest
+    steps:
+        - name: 'With prefix'
+          env:
+              STRVAR_ONE: '1'
+              STRVAR_TWO: '2'
+              ONE: '3'
+          uses: AndreasNel/string-vars-action@master
+          with:
+              prefix: 'STRVAR_'
+              instring: '{ONE} {ONE} {TWO}'
+        - name: 'Without prefix'
+          env:
+              ONE: '1'
+              TWO: '2'
+          uses: AndreasNel/string-vars-action@master
+          with:
+              instring: '{ONE} {ONE} {TWO}'
+```
+
+In both cases, the string `1 1 2` will be the output.

action.yml

@@ -8,7 +8,7 @@ inputs:
     # Add this if Github starts allowing structured data for inputs.
     # variables:
     #     description: 'Key value pairs of the variable name and the value that it should have.'
-    #     required: true
+    #     required: false
     instring:
         description: 'The string containing the variable names.'
         required: true