Documentation enhancements (#73)

docs/concepts.md

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+# Concepts
+- [Matrix](#matrix)
+- [mxisd](#mxisd)
+
+## Matrix
+The following concepts are part of the Matrix ecosystem and specification.
+
+### 3PID
+`3PID` stands for Third-Party Identifier.  
+It is also commonly written:
+- `3pid`
+- `tpid`
+
+A 3PID is a globally unique canonical identifier which is made of:
+- Medium, which describes what network it belongs to (Email, Phone, Twitter, Discord, etc.)
+- Address, the actual value people typically use on a daily basis.
+
+mxisd core mission is to map those identifiers to Matrix User IDs.
+
+### Homeserver
+Where a user **account and data** are stored.
+
+### Identity server
+An Identity server:
+- Does lookup of 3PIDs to User Matrix IDs.
+- Does validate 3PIDs ownership, typically by sending a code that the user has to enter in an application/on a website.
+- Does send notifications about room invites where no Matrix User ID could be found for the invitee.
+
+An Identity server:
+- **DOES NOT** store user accounts.
+- **DOES NOT** store user data.
+- **DOES NOT** allow migration of user account and/or data between homeservers. 
+
+### 3PID session
+The fact to validate a 3PID (email, phone number, etc.) via the introduction of a token which was sent to the 3PID address.
+
+## mxisd
+The following concepts are specific to mxisd.
+
+### Identity store
+Where your user accounts and 3PID mappings are stored.
+
+mxisd itself **DOES NOT STORE** user accounts or 3PID mappings.