Add reworked configuration docs. (#245)

* Add reworked configuration docs.

* Align the table.

* Address review comments.

* Address review.

docs/configuration/description.md

@@ -0,0 +1,70 @@
+# Configuration
+
+Files can be put in two places.
+
+* `${HOME}/.config/mainframer` or
+  [`${XDG_CONFIG_HOME}`](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html)
+  if available — the global configuration;
+* `.mainframer` — the project configuration.
+
+It is posssible to declare common configuration using the global one
+and fine-tune it using the project one.
+
+## `.mainframer`
+
+The directory contains following files.
+
+Name         | Required | Keep in VCS | Description
+-------------|----------|-------------|------------
+`config.yml` | Yes      | No          | Configuration options.
+`ignore.yml` | No       | Yes         | Ignore rules for copying files.
+
+## `.mainframer/config.yml`
+
+```yaml
+remote:
+  name: "{SSH machine name}"
+push:
+  compression: {level}
+pull:
+  compression: {level}
+```
+
+Name               | Required | Value   | Default | Description
+-------------------|----------|---------|---------|------------------
+`remote.name`      | Yes      | `string`| —       | Remote machine name from SSH config or hostname / IP address.
+`push.compression` | No       | `0..9`  | `0`     | Compression level used to copy files from local machine to remote one.
+`pull.compression` | No       | `0..9`  | `0`     | Compression level used to copy files from remote machine to local one.
+
+Compression level is inherited from underlying `rsync`
+which uses [`zlib` values](https://www.zlib.net/manual.html):
+
+> The compression level must be between `0` and `9`:
+> `1` gives best speed, `9` gives best compression, `0` gives no compression at all
+> (the input data is simply copied a block at a time).
+
+## `.mainframer/ignore.yml`
+
+```yaml
+push:
+  - "pattern"
+  - "..."
+pull:
+  - "pattern"
+  - "..."
+both:
+  - "pattern"
+  - "..."
+```
+
+Name   | Description                                                   | Suggestions
+-------|---------------------------------------------------------------|------------
+`push` | Patterns used to copy files from local machine to remote one. | Build results.
+`pull` | Patterns used to copy files from remote machine to local one. | Source code.
+`both` | Patterns used to copy files both ways.                        | VCS, IDE-related directories.
+
+Ignore patterns are inherited from underlying `rsync`.
+Please refer to [`rsync` documentation](https://download.samba.org/pub/rsync/rsync.html) —
+see the _Include/Exclude Pattern Rules_ section.
+The format is very similar to `.gitignore`.
+