Add oauth2 documentation (#6604)

* Add oauth2 documentation

Signed-off-by: Jonas Franz <info@jonasfranz.software>

* Apply suggestions from code review

Co-Authored-By: jonasfranz <info@jonasfranz.software>

* Update docs/content/doc/advanced/oauth2-provider.md

Co-Authored-By: jonasfranz <info@jonasfranz.software>

* Update oauth2-provider.md

3 files changed, 100 insertions, 0 deletions

diff --git a/docs/content/doc/advanced/api-usage.en-us.md b/docs/content/doc/advanced/api-usage.en-us.md
index d5a7b3a6ef..c5db817fdb 100644
--- a/docs/content/doc/advanced/api-usage.en-us.md
+++ b/docs/content/doc/advanced/api-usage.en-us.md
@@ -39,6 +39,14 @@ Gitea parses queries and headers to find the token in
 You can create an API key token via your Gitea installation's web interface:
 `Settings | Applications | Generate New Token`.
 
+### OAuth2
+
+Access tokens obtained from Gitea's [OAuth2 provider](https://docs.gitea.io/en-us/oauth2-provider) are accepted by these methods:
+
+- `Authorization bearer ...` header in HTTP headers
+- `token=...` parameter in URL query string
+- `access_token=...` parameter in URL query string
+
 ### More on the `Authorization:` header
 
 For historical reasons, Gitea needs the word `token` included before
diff --git a/docs/content/doc/advanced/oauth2-provider.md b/docs/content/doc/advanced/oauth2-provider.md
new file mode 100644
index 0000000000..c924d1ce07
--- /dev/null
+++ b/docs/content/doc/advanced/oauth2-provider.md
@@ -0,0 +1,92 @@
+---
+date: "2019-04-19:44:00+01:00"
+title: "OAuth2 provider"
+slug: "oauth2-provider"
+weight: 41
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "advanced"
+    name: "OAuth2 Provider"
+    weight: 41
+    identifier: "oauth2-provider"
+---
+
+
+# OAuth2 provider
+
+Gitea supports acting as an OAuth2 provider to allow third party applications to access its resources with the user's consent. This feature is available since release 1.8.0.
+
+## Endpoints
+
+
+Endpoint               | URL
+-----------------------|----------------------------
+Authorization Endpoint | `/login/oauth/authorize`
+Access Token Endpoint  | `/login/oauth/access_token`
+
+
+## Supported OAuth2 Grants
+
+At the moment Gitea only supports the [**Authorization Code Grant**](https://tools.ietf.org/html/rfc6749#section-1.3.1) standard with additional support of the [Proof Key for Code Exchange (PKCE)](https://tools.ietf.org/html/rfc7636) extension.
+ 
+
+To use the Authorization Code Grant as a third party application it is required to register a new application via the "Settings" (`/user/settings/applications`) section of the settings.
+
+## Scopes
+
+Currently Gitea does not support scopes (see [#4300](https://github.com/go-gitea/gitea/issues/4300)) and all third party applications will be granted access to all resources of the user and his/her organizations.
+
+## Example
+
+**Note:** This example does not use PKCE.
+
+1. Redirect to user to the authorization endpoint in order to get his/her consent for accessing the resources:
+
+```curl
+https://[YOUR-GITEA-URL]/login/oauth/authorize?client_id=CLIENT_ID&redirect_uri=REDIRECT_URI& response_type=code&state=STATE
+``` 
+
+The `CLIENT_ID` can be obtained by registering an application in the settings. The `STATE` is a random string that will be send back to your application after the user authorizes. The `state` parameter is optional but should be used to prevent CSRF attacks.
+
+
+![Authorization Page](/authorize.png)
+
+The user will now be asked to authorize your application. If they authorize it, the user will be redirected to the `REDIRECT_URL`, for example:
+
+```curl
+https://[REDIRECT_URI]?code=RETURNED_CODE&state=STATE
+```
+
+2. Using the provided `code` from the redirect, you can request a new application and refresh token. The access token endpoints accepts POST requests with  `application/json` and `application/x-www-form-urlencoded` body, for example:
+
+```curl
+POST https://[YOUR-GITEA-URL]/login/oauth/access_token
+```
+
+```json
+{
+	"client_id": "YOUR_CLIENT_ID",
+	"client_secret": "YOUR_CLIENT_SECRET",
+	"code": "RETURNED_CODE",
+	"grant_type": "authorization_code",
+	"redirect_uri": "REDIRECT_URI"
+}
+```
+
+Response:
+```json
+{  
+"access_token":"eyJhbGciOiJIUzUxMiIsInR5cCI6IkpXVCJ9.eyJnbnQiOjIsInR0IjowLCJleHAiOjE1NTUxNzk5MTIsImlhdCI6MTU1NTE3NjMxMn0.0-iFsAwBtxuckA0sNZ6QpBQmywVPz129u75vOM7wPJecw5wqGyBkmstfJHAjEOqrAf_V5Z-1QYeCh_Cz4RiKug",  
+"token_type":"bearer",  
+"expires_in":3600,  
+"refresh_token":"eyJhbGciOiJIUzUxMiIsInR5cCI6IkpXVCJ9.eyJnbnQiOjIsInR0IjoxLCJjbnQiOjEsImV4cCI6MTU1NzgwNDMxMiwiaWF0IjoxNTU1MTc2MzEyfQ.S_HZQBy4q9r5SEzNGNIoFClT43HPNDbUdHH-GYNYYdkRfft6XptJBkUQscZsGxOW975Yk6RbgtGvq1nkEcklOw"  
+}
+```
+
+The `CLIENT_SECRET` is the unique secret code generated for this application. Please note that the secret will only be visible after you created/registered the application with Gitea and cannot be recovered. If you lose the secret you must regenerate the secret via the application's settings.
+
+The `REDIRECT_URI` in the `access_token` request must match the `REDIRECT_URI` in the `authorize` request.
+
+3. Use the  `access_token` to make [API requests](https://docs.gitea.io/en-us/api-usage#oauth2) to access the user's resources.
diff --git a/docs/static/authorize.png b/docs/static/authorize.png
new file mode 100644
index 0000000000..7556b1220c
--- /dev/null
+++ b/docs/static/authorize.png