Add optional packer plugins hook (nvim-lua#20)

* Add optional packer plugins hook

* Provide help in README and simplify module

* Fix spelling and tweak verbiage about after/plugin

README.md

@@ -18,6 +18,37 @@ This repo is meant to be used as a starting point for a user's own configuration
 * Start Neovim (`nvim`) and run `:PackerInstall` - ignore any error message about missing plugins, `:PackerInstall` will fix that shortly
 * Restart Neovim
 
+### Configuration
+
+You could directly modify the `init.lua` file with your personal customizations. This option is the most straightforward, but if you update your config from this repo, you may need to reapply your changes.
+
+An alternative approach is to create a separate `custom.plugins` module to register your own plugins. In addition, you can handle further customizations in the `/after/plugin/` directory (see `:help load-plugins`). See the following examples for more information. Leveraging this technique should make upgrading to a newer version of this repo easier. 
+
+#### Example `plugins.lua`
+
+The following is an example of a `plugins.lua` module (located at `$HOME/.config/nvim/lua/custom/plugins.lua`) where you can register your own plugins. 
+
+```lua
+return function(use)
+  use({
+    "folke/which-key.nvim",
+      config = function()
+        require("which-key").setup({})
+      end
+  })
+end
+```
+
+#### Example `defaults.lua`
+
+For further customizations, you can add a file in the `/after/plugin/` folder (see `:help load-plugins`) to include your own options, keymaps, autogroups, and more. The following is an example `defaults.lua` file (located at `$HOME/.config/nvim/after/plugin/defaults.lua`).
+
+```lua
+vim.opt.relativenumber = true
+
+vim.keymap.set('n', '<leader>sr', require('telescope.builtin').resume, { desc = '[S]earch [R]esume' })
+```
+
 ### Contribution
 
 Pull-requests are welcome. The goal of this repo is not to create a Neovim configuration framework, but to offer a starting template that shows, by example, available features in Neovim. Some things that will not be included:

init.lua

@@ -57,6 +57,10 @@ require('packer').startup(function(use)
   -- Fuzzy Finder Algorithm which requires local dependencies to be built. Only load if `make` is available
   use { 'nvim-telescope/telescope-fzf-native.nvim', run = 'make', cond = vim.fn.executable 'make' == 1 }
 
+  -- Add custom plugins to packer from /nvim/lua/custom/plugins.lua
+  local has_plugins, plugins = pcall(require, "custom.plugins")
+  if has_plugins then plugins(use) end
+
   if is_bootstrap then
     require('packer').sync()
   end