Add docs for full feature list

Describe what Gitlab Shell is capable of

README.md

@@ -18,6 +18,8 @@ An overview of the four cases described above:
 1. git pull over SSH -> gitlab-shell -> API call to gitlab-rails (Authorization) -> accept or decline -> establish Gitaly session
 1. git push over SSH -> gitlab-shell (git command is not executed yet) -> establish Gitaly session -> (in Gitaly) gitlab-shell pre-receive hook -> API call to gitlab-rails (authorization) -> accept or decline push
 
+[Full feature list](/doc/features.md)
+
 ## Code status
 
 [![pipeline status](https://gitlab.com/gitlab-org/gitlab-shell/badges/main/pipeline.svg)](https://gitlab.com/gitlab-org/gitlab-shell/-/pipelines?ref=main)
@@ -94,10 +96,6 @@ If no `GITALY_CONNECTION_INFO` is set, the test suite will still run, but any
 tests requiring Gitaly will be skipped. They will always run in the CI
 environment.
 
-## Git LFS
-
-Starting with GitLab 8.12, GitLab supports Git LFS authentication through SSH.
-
 ## Logging Guidelines
 
 In general, it should be possible to determine the structure, but not content,

doc/features.md

+### Discover
+
+Allows users to identify themselves on an instance via SSH. The command is useful for checking out quickly whether a user has SSH access to the instance:
+
+```bash
+ssh git@<hostname>
+
+PTY allocation request failed on channel 0
+Welcome to GitLab, @username!
+Connection to staging.gitlab.com closed.
+```
+
+When permission is denied:
+
+```bash
+ssh git@<hostname>
+git@<hostname>: Permission denied (publickey).
+```
+
+### Git operations
+
+Gitlab Shell provides support for Git operations over SSH via processing `git-upload-pack`, `git-receive-pack` and `git-upload-archive` SSH commands. It limit the set of commands to predefined git commands (git push, git clone/pull, git archive).
+
+### Generate new 2FA recovery codes
+
+Allows users to [generate new 2FA recovery codes](https://docs.gitlab.com/ee/user/profile/account/two_factor_authentication.html#generate-new-recovery-codes-using-ssh).
+
+```bash
+ssh git@<hostname> 2fa_recovery_codes
+Are you sure you want to generate new two-factor recovery codes?
+Any existing recovery codes you saved will be invalidated. (yes/no)
+yes
+
+Your two-factor authentication recovery codes are:
+...
+```
+
+### Verify 2FA OTP
+
+Allows users to [verify their 2FA OTP](https://docs.gitlab.com/ee/security/two_factor_authentication.html#2fa-for-git-over-ssh-operations).
+
+```bash
+ssh git@<hostname> 2fa_verify
+OTP: 347419
+
+OTP validation failed.
+```
+
+### LFS authentication
+
+Allows users to generate credentials for LFS authentication.
+
+```bash
+ssh git@<hostname> git-lfs-authenticate <project-path> <upload/download>
+
+{"header":{"Authorization":"Basic ..."},"href":"https://gitlab.com/user/project.git/info/lfs","expires_in":7200}
+```
+
+### Personal access token
+
+Allows users to personal access tokens via SSH
+
+```bash
+ssh git@<hostname> personal_access_token <name> <scope1[,scope2,...]> [ttl_days]
+
+Token:   glpat-...
+Scopes:  api
+Expires: 2022-02-05
+```