Add issue template [ci skip]

I often find myself asking for more information about an issue and
especially for ways to reproduce bugs. This template and sample
reproduction script will hopefully help us get more information from the
beginning.

.github/ISSUE_TEMPLATE.md

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+### Steps to reproduce
+<!-- Be as specific as possible. A [reproduction script][] or sample application
+to reproduce the problem are especially helpful -->
+
+[reproduction script]: https://github.com/thoughtbot/factory_bot/blob/master/.github/REPRODUCTION_SCRIPT.rb
+
+### Expected behavior
+<!-- Tell us what should happen -->
+
+### Actual behavior
+<!-- Tell us what happens instead -->
+
+### System configuration
+**factory_bot version**:  
+**rails version**:  
+**ruby version**:  

.github/REPRODUCTION_SCRIPT.rb

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+require "bundler/inline"
+
+gemfile(true) do
+  source "https://rubygems.org"
+  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
+  gem "factory_bot", "~> 5.0"
+end
+
+require "factory_bot"
+require "minitest/autorun"
+
+class Post
+  attr_accessor :body
+end
+
+FactoryBot.define do
+  factory :post do
+    body { "Post body" }
+  end
+end
+
+class FactoryBotTest < Minitest::Test
+  def test_factory_bot_stuff
+    body_override = "Body override"
+
+    post = FactoryBot.build(:post, body: body_override)
+
+    assert_equal post.body, body_override
+  end
+end