Pull request: all: add a changelog and a developer guide

Merge in DNS/adguard-home from 2294-changelog to master

Closes #2294.

Squashed commit of the following:

commit 25ffc9786d23d83448b916a81f0b20135b051169
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Nov 12 19:40:48 2020 +0300

    all: add a changelog and a developer guide

CHANGELOG.md

@@ -0,0 +1,15 @@
+# AdGuard Home Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on
+[*Keep a Changelog*](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to
+[Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- This changelog :-) (#2294).
+- `HACKING.md`, a guide for developers.

HACKING.md

@@ -0,0 +1,149 @@
+ #  AdGuardHome Developer Guidelines
+
+As of **2020-11-12**, 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.
+
+##  Git
+
+ *  Follow the commit message header format:
+
+    ```none
+    pkg: fix the network error logging issue
+    ```
+
+    Where `pkg` is the package where most changes took place.  If there are
+    several such packages, just write `all`.
+
+ *  Keep your commit messages to be no wider than eighty (**80**) columns.
+
+ *  Only use lowercase letters in your commit message headers.
+
+##  Go
+
+ *  <https://github.com/golang/go/wiki/CodeReviewComments>.
+
+ *  <https://github.com/golang/go/wiki/TestComments>.
+
+ *  <https://go-proverbs.github.io/>
+
+ *  Avoid `init` and use explicit initialization functions instead.
+
+ *  Avoid `new`, especially with structs.
+
+ *  Document everything, including unexported top-level identifiers, to build
+    a habit of writing documentation.
+
+ *  Don't use underscores in file and package names, unless they're build tags
+    or for tests.  This is to prevent accidental build errors with weird tags.
+
+ *  Don't write code with more than four (**4**) levels of indentation.  Just
+    like [Linus said], plus an additional level for an occasional error check or
+    struct initialization.
+
+ *  Eschew external dependencies, including transitive, unless
+    absolutely necessary.
+
+ *  No `goto`.
+
+ *  No shadowing, since it can often lead to subtle bugs, especially with
+    errors.
+
+ *  Prefer constants to variables where possible.  Reduce global variables.  Use
+    [constant errors] instead of `errors.New`.
+
+ *  Put comments above the documented entity, **not** to the side, to improve
+    readability.
+
+ *  Use `gofumpt --extra -s`.
+
+    **TODO(a.garipov):** Add to the linters.
+
+ *  Use linters.
+
+ *  Use named returns to improve readability of function signatures.
+
+ *  When a method implements an interface, start the doc comment with the
+    standard template:
+
+    ```go
+    // Foo implements the Fooer interface for *foo.
+    func (f *foo) Foo() {
+        // …
+    }
+    ```
+
+ *  Write logs and error messages in lowercase only to make it easier to `grep`
+    logs and error messages without using the `-i` flag.
+
+ *  Write slices of struct like this:
+
+    ```go
+    ts := []T{{
+            Field: Value0,
+            // …
+    }, {
+            Field: Value1,
+            // …
+    }, {
+            Field: Value2,
+            // …
+    }}
+    ```
+
+[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
+
+##  Text, Including Comments
+
+ *  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.
+
+    The only exception are long hyperlinks.
+
+ *  Use U.S. English, as it is the most widely used variety of English in the
+    code right now as well as generally.
+
+ *  Use double spacing between sentences to make sentence borders more clear.
+
+ *  Use the serial comma (a.k.a. Oxford comma) to improve comprehension,
+    decrease ambiguity, and use a common standard.
+
+ *  Write todos like this:
+
+    ```go
+    // TODO(usr1): Fix the frobulation issue.
+    ```
+
+    Or, if several people need to look at the code:
+
+    ```go
+    // TODO(usr1, usr2): Fix the frobulation issue.
+    ```
+
+##  Markdown
+
+ *  **TODO(a.garipov):** Define our Markdown conventions.
+
+##  YAML
+
+ *  **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.
+
+ *  No extra indentation in multiline arrays:
+
+    ```yaml
+    'values':
+    - 'value-1'
+    - 'value-2'
+    - 'value-3'
+    ```
+
+ *  Prefer single quotes for string to prevent accidental escaping, unless
+    escaping is required.
+
+[NO-rway Law]: https://news.ycombinator.com/item?id=17359376