[Docs] Clone filters (#14555)

In summary: set uploadpack.allowfilter

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>

docs/content/doc/advanced/clone-filter.en-us.md

@@ -0,0 +1,65 @@
+---
+date: "2021-02-02"
+title: "Clone filters (partial clone)"
+slug: "clone-filters"
+weight: 25
+draft: false
+toc: false
+menu:
+  sidebar:
+    parent: "advanced"
+    name: "Clone filters"
+    weight: 25
+    identifier: "clone-filters"
+---
+
+# Clone filters (partial clone)
+
+Git introduces `--filter` option to `git clone` command, which filters out
+large files and objects (such as blobs) to create partial clone of a repo.
+Clone filters are especially useful for large repo and/or metered connection,
+where full clone (without `--filter`) can be expensive (as all history data
+must be downloaded).
+
+This requires Git version 2.22 or later, both on the Gitea server and on the
+client. For clone filters to work properly, make sure that Git version
+on the client is at least the same as on the server (or later). Login to
+Gitea server as admin and head to Site Administration -> Configuration to
+see Git version of the server.
+
+By default, clone filters are disabled, which cause the server to ignore
+`--filter` option.
+
+To enable clone filters on per-repo basis, edit the repo's `config` on
+repository location. Consult `ROOT` option on `repository` section of
+Gitea configuration (`app.ini`) for the exact location. For example, to
+enable clone filters for `some-repo`, edit
+`/var/gitea/data/gitea-repositories/some-user/some-repo.git/config` and add:
+
+```ini
+[uploadpack]
+	allowfilter = true
+```
+
+To enable clone filters globally, add that config above to `~/.gitconfig`
+of user that run Gitea (for example `git`).
+
+Alternatively, you can use `git config` to set the option.
+
+To enable for a specific repo:
+
+```bash
+cd /var/gitea/data/gitea-repositories/some-user/some-repo.git
+git config --local uploadpack.allowfilter true
+```
+To enable globally, login as user that run Gitea and:
+
+```bash
+git config --global uploadpack.allowfilter true
+```
+
+See [GitHub blog post: Get up to speed with partial clone](https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/)
+for common use cases of clone filters (blobless and treeless clones), and
+[Gitlab docs for partial clone](https://docs.gitlab.com/ee/topics/git/partial_clone.html)
+for more advanced use cases (such as filter by file size and remove
+filters to turn partial clone into full clone).