📝 Created a full README with instructions

README.md

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 # hexo-custom-fields
 A Hexo plugin that allows the default Markdown source files to be used as custom fields
+
+## Motive
+
+I am a front-end developer originally. I used many CMS throughout
+the years and with many of them it was common to have custom fields.
+Which is a functionality that allows the coder to be able to include just snippets
+of text or an image path into a template.
+I wanted to try a static-site generator more deeply and chose Hexo which I like so far.
+It is just missing an easy way to include custom fields directly from
+your Markdown source files.
+That's exactly what this plugin is trying to solve!
+
+## Installation
+
+NPM
+```
+npm install --save hexo-custom-fields
+```
+
+Yarn
+```
+yarn add hexo-custom-fields
+```
+
+## Usage
+
+Insert the `custom_field()` tag in your templates (in this example with EJS):
+
+```
+<%- custom_field('posts|pages, 'title_in_the_front_matter', 'name_of_the_field') %>
+```
+
+Argument | Description
+-------- | -----------
+first     | 'posts' or 'pages' - this determines if the directory 'source/_posts' or 'source/' is used for that field - [Read more about the location in the Hexo documentation](https://hexo.io/docs/writing.html#Layout)
+second     |  'title' value in the front-matter of the Markdown source file - [Read more about the front-matter in the Hexo documentation](https://hexo.io/docs/front-matter.html)
+third   |  the actual name of the chosen field. Below this table there is another one with possible options.
+
+
+**One can use the default fields provided by [Hexo](https://hexo.io/docs/variables.html#Page-Variables)**.
+**The most important options for the third argument are these:**
+
+Option   | Description
+-------- | -----------
+'any_field_name'    |  You can use an unlimited amount of custom fields in the front-matter of a source file. You can directly get these with their respective names - *string*
+'content'           |  The rendered content of the Markdown file (so everything after the front-matter) - *HTML*
+'title'             |  The given title - *string*
+'date'              |  The date and time the source file was created at - *formatted date (with timezone)*
+'updated'           |  The date and time the source file was last updated at - *formatted date (with timezone)*
+'permalink'         |  The full URL of the post or page - *URL*
+'_id'               |  The unique ID Hexo assigned to the source file - *string*
+
+
+**Example:**
+
+Display the custom field *'birthday'*
+from the Markdown source file *'front-page'*
+in the 'source' directory (which makes it a *'page'*)
+[Read more about the location in the Hexo documentation](https://hexo.io/docs/writing.html#Layout)
+```
+<%- custom_field('pages', 'front-page', 'birthday') %>
+```
+
+
+License
+-------
+
+[MIT](LICENSE)