Implement renderInstructions for themer-slack

cli/packages/themer-slack/README.md

@@ -12,6 +12,4 @@ Then pass `themer-slack` as a `-t` (`--template`) arg to `themer`:
 
     themer -c my-colors.js -t themer-slack -o gen
 
-## Output
-
-`themer-slack` will generate a `themer-slack-dark.txt` or `themer-slack-light.txt` file (or both), depending on the color set you passed to themer. Simply copy the contents of this file and paste into the custom theme input in Slack's preferences.
+Installation instructions for the generated theme(s) will be included in `<output dir>/README.md`.

cli/packages/themer-slack/lib/__snapshots__/index.spec.js.snap

@@ -0,0 +1,7 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`renderInstructions should provide installation instructions 1`] = `
+"
+Copy the contents of \`themer-slack-dark.txt\` or \`themer-slack-light.txt\` and paste into the custom theme input in Slack's preferences.
+"
+`;

cli/packages/themer-slack/lib/index.js

@@ -14,6 +14,11 @@ const render = colors => Object.entries(colors).map(
   })
 );
 
+const renderInstructions = paths => `
+Copy the contents of ${paths.map(p => `\`${p}\``).join(' or ')} and paste into the custom theme input in Slack's preferences.
+`;
+
 module.exports = {
   render,
+  renderInstructions,
 };

cli/packages/themer-slack/lib/index.spec.js

@@ -1,4 +1,4 @@
-const { render } = require('./index'),
+const { render, renderInstructions } = require('./index'),
   { colors } = require('../../themer-colors-default');
 
 describe('render', () => {
@@ -20,3 +20,11 @@ describe('render', () => {
   });
 
 });
+
+describe('renderInstructions', () => {
+  it('should provide installation instructions', async () => {
+    const files = await Promise.all(render(colors));
+    const instructions = renderInstructions(files.map(({ name }) => name));
+    expect(instructions).toMatchSnapshot();
+  });
+});