Markdownlint (#183)

Add default linting rules for Markdown https://github.com/DavidAnson/markdownlint

I met a bunch of warnings in my editor (VSCode) when opening the project to work on #181
This PR fixes them for Markdown, also with some rules for Markdownlint, one of the most common linters for this language

Co-authored-by: Greg Bowler <greg.bowler@g105b.com>

.markdownlint.json

@@ -0,0 +1,18 @@
+{
+	"default": true,
+	"heading-style": {
+		"style": "setext_with_atx"
+	},
+	"line-length": false,
+	"no-hard-tabs": false,
+	"no-inline-html": {
+		"allowed_elements": ["a", "br", "img", "kbd"]
+	},
+	"no-multiple-blanks": {
+		"maximum": 2
+	},
+	"ul-indent": false,
+	"ul-style": {
+		"style": "consistent"
+	}
+}

.markdownlintignore

@@ -0,0 +1 @@
+vendor/

CONTRIBUTING.md

@@ -74,11 +74,11 @@ If you are looking for a project to contribute to, we promote the use of "good f
 Once you've identified what you want to contribute to, the process of making a change to the code and updating the repository is as follows:
 
 1. [Fork the repository](https://help.github.com/articles/fork-a-repo/) so you own a copy of it.
-1. [Clone your forked repository](https://help.github.com/articles/cloning-a-repository/) to your computer.
-2. Preferably, write a unit test to cover your contribution.
-3. Make the changes to the code.
-4. [Commit and push](https://help.github.com/articles/adding-a-file-to-a-repository-using-the-command-line/) your changes back to your forked repository.
-5. [Submit a pull request](https://help.github.com/articles/creating-a-pull-request/) to the original repository at PHP.Gt.
+2. [Clone your forked repository](https://help.github.com/articles/cloning-a-repository/) to your computer.
+3. Preferably, write a unit test to cover your contribution.
+4. Make the changes to the code.
+5. [Commit and push](https://help.github.com/articles/adding-a-file-to-a-repository-using-the-command-line/) your changes back to your forked repository.
+6. [Submit a pull request](https://help.github.com/articles/creating-a-pull-request/) to the original repository at PHP.Gt.
 
 #### Submitting a pull request (PR)
 
@@ -106,7 +106,7 @@ PHP.Gt is open source and free, and always will be. If you are interested in spo
 Coding style guide
 ------------------
 
-A guide (not a rulebook) is available at https://php.gt/styleguide, which allows for developers to code in a consistent style across all repositories. It is recommended to adhere to the style guide when creating pull requests.
+A guide (not a rulebook) is available at <https://php.gt/styleguide>, which allows for developers to code in a consistent style across all repositories. It is recommended to adhere to the style guide when creating pull requests.
 
 Test driven development (TDD)
 -----------------------------

README.md

@@ -1,5 +1,5 @@
-Translate CSS selectors to XPath queries.
-=========================================
+Translate CSS selectors to XPath queries
+========================================
 
 A lightweight and dependency free CSS to XPath translator. This repository is used to bring modern DOM functionality like [`querySelectorAll()`][qsa] to PHP in the [PHP.Gt/Dom][gt-dom] project.