Pull request: all: reformat yaml, add yaml formatting standard

Merge in DNS/adguard-home from 2297-yaml to master

Closes #2297.

Squashed commit of the following:

commit 85df3a38a14adb1965944ddf14b197c12a213057
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Nov 20 17:52:22 2020 +0300

    all: improve HACKING.md

commit 079acdfe41cc12ab6aa13d7c28dcbf7b7b3c8380
Merge: 202ea078e 3045da174
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Nov 20 17:50:34 2020 +0300

    Merge branch 'master' into 2297-yaml

commit 202ea078e29d88871a32ac6e668dfae6db802bab
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Nov 12 20:25:42 2020 +0300

    all: reformat yaml, add yaml formatting standard

HACKING.md

@@ -1,9 +1,11 @@
  #  AdGuardHome Developer Guidelines
 
-As of **2020-11-12**, this document is still a work-in-progress.  Some of the
+As of **2020-11-20**, this document is still a work-in-progress.  Some of the
 rules aren't enforced, and others might change.  Still, this is a good place to
 find out about how we **want** our code to look like.
 
+The rules are mostly sorted in the alphabetical order.
+
 ##  Git
 
  *  Follow the commit message header format:
@@ -13,11 +15,14 @@ find out about how we **want** our code to look like.
     ```
 
     Where `pkg` is the package where most changes took place.  If there are
-    several such packages, just write `all`.
+    several such packages, or the change is top-level only, write `all`.
 
- *  Keep your commit messages to be no wider than eighty (**80**) columns.
+ *  Keep your commit messages, including headers, to eighty (**80**) columns.
 
- *  Only use lowercase letters in your commit message headers.
+ *  Only use lowercase letters in your commit message headers.  The rest of the
+    message should follow the plain text conventions below.
+
+    The only exception are direct mentions of identifiers from the source code.
 
 ##  Go
 
@@ -98,8 +103,20 @@ find out about how we **want** our code to look like.
 [constant errors]: https://dave.cheney.net/2016/04/07/constant-errors
 [Linus said]:      https://www.kernel.org/doc/html/v4.17/process/coding-style.html#indentation
 
+##  Markdown
+
+ *  **TODO(a.garipov):** Define our Markdown conventions.
+
 ##  Text, Including Comments
 
+ *  End sentences with appropriate punctuation.
+
+ *  Headers should be written with all initial letters capitalized, except for
+    references to variable names that start with a lowercase letter.
+
+ *  Start sentences with a capital letter, unless the first word is a reference
+    to a variable name that starts with a lowercase letter.
+
  *  Text should wrap at eighty (**80**) columns to be more readable, to use
     a common standard, and to allow editing or diffing side-by-side without
     wrapping.
@@ -126,17 +143,17 @@ find out about how we **want** our code to look like.
     // TODO(usr1, usr2): Fix the frobulation issue.
     ```
 
-##  Markdown
-
- *  **TODO(a.garipov):** Define our Markdown conventions.
-
 ##  YAML
 
+ *  **TODO(a.garipov):** Define naming conventions for schema names in our
+    OpenAPI YAML file.  And just generally OpenAPI conventions.
+
  *  **TODO(a.garipov):** Find a YAML formatter or write our own.
 
  *  All strings, including keys, must be quoted.  Reason: the [NO-rway Law].
 
- *  Indent with two (**2**) spaces.
+ *  Indent with two (**2**) spaces.  YAML documents can get pretty
+    deeply-nested.
 
  *  No extra indentation in multiline arrays:
 
@@ -147,7 +164,10 @@ find out about how we **want** our code to look like.
     - 'value-3'
     ```
 
- *  Prefer single quotes for string to prevent accidental escaping, unless
-    escaping is required.
+ *  Prefer single quotes for strings to prevent accidental escaping, unless
+    escaping is required or there are single quotes inside the string (e.g. for
+    *GitHub Actions*).
+
+ *  Use `>` for multiline strings, unless you need to keep the line breaks.
 
 [NO-rway Law]: https://news.ycombinator.com/item?id=17359376