docs: added permissions guide

Author: Maksym Mykhailenko

docs/guides/permissions-guide/images/project-roles.png

@@ -0,0 +1,77 @@
+# Permissions Guide
+
+What kind of permissions we have, how they work and how to use them.
+
+- [Permissions Guide](#permissions-guide)
+  - [Overview](#overview)
+    - [Topcoder Roles](#topcoder-roles)
+    - [Project Role](#project-role)
+  - [How to Use](#how-to-use)
+  - [References](#references)
+
+## Overview
+
+Every user may have 2 kind of roles: **Topcoder Roles** and **Project Role**.
+
+### Topcoder Roles
+
+These roles are assigned to user accounts. User may have several **Topcoder Roles**. See [the list of all Topcoder Roles](https://github.com/topcoder-platform/tc-project-service/blob/develop/src/constants.js#L55-L69) which we use in Topcoder Project Service.
+
+<img src="./images/topcoder-roles.png" width="689">
+
+By default every user has one role `Topcoder User`, generally this means that such a user is either **customer** or **community member** (freelancer).
+
+### Project Role
+
+When user joins some project and become a member of the project, such a user has one **Project Role** inside that project. One user may have different **Project Role** in different projects. See [the list of all Project Roles](https://github.com/topcoder-platform/tc-project-service/blob/develop/src/constants.js#L23-L33) which we use in Topcoder Project Service.
+
+<img src="./images/project-roles.png" width="411">
+
+## How to Use
+
+Let's say you would like to add a new place in code where you want to check user roles/permissions. Please, follow the next guide:
+
+1. Check if we already have defined permission for your case in the [permissions list](https://htmlpreview.github.io/?https://github.com/topcoder-platform/tc-project-service/blob/develop/docs/permissions.html).
+
+2. If you cannot find the permission you need, add new permission to the file https://github.com/topcoder-platform/tc-project-service/blob/develop/src/permissions/constants.js.
+
+   - Follow the guides on how to add a new permission in the header of this file.
+
+3. After you add a new permission, regenerate [permissions list](https://htmlpreview.github.io/?https://github.com/topcoder-platform/tc-project-service/blob/develop/docs/permissions.html) by running `npm run generate:doc:permissions`.
+
+4. There are 2 places where you would usually check permissions:
+   1. Check if user can call some endpoint (https://github.com/topcoder-platform/tc-project-service/blob/develop/src/permissions/index.js):
+
+      ```js
+      Authorizer.setPolicy('projectMember.view', generalPermission(PERMISSION.READ_PROJECT_MEMBER));
+      ```
+
+      or
+
+      ```js
+      Authorizer.setPolicy('projectMember.edit', generalPermission([
+         PERMISSION.UPDATE_PROJECT_MEMBER_CUSTOMER,
+         PERMISSION.UPDATE_PROJECT_MEMBER_NON_CUSTOMER,
+      ]));
+      ```
+
+   2. Inside some endpoint code:
+
+      ```js
+      import util from '../util';
+      import { PERMISSION } from '../permissions/constants';
+
+      (req, res, next) => {
+         ...
+         if (hasPermissionByReq(permission, req)) {
+            ...
+         }
+         ...
+      }
+      ```
+
+## References
+
+- [Permissions list](https://htmlpreview.github.io/?https://github.com/topcoder-platform/tc-project-service/blob/develop/docs/permissions.html)
+
+- [Permissions list source](https://github.com/topcoder-platform/tc-project-service/blob/develop/src/permissions/constants.js)