docs: add instagram data source doc

data-sources/docs/instagram.md

@@ -0,0 +1,53 @@
+# Instagram
+
+Users can connect their Instagram account by posting a link to the verification data inside the biography description.  
+The verification data must be a JSON object formed as described inside ["Verification data"](../../README.md#verification-data).
+
+## Example biography
+
+An example biography can be found [here](https://www.instagram.com/test_desmos_user).
+
+## Verification process
+
+The verification process on Instagram is made of the following steps:
+
+1. The user uploads their verification data to an online storage (eg. PasteBin).
+2. The user links the online storage URL to their account by putting it inside their account biography.
+3. The user requests Themis to cache their user profile data by an access token with the permission requesting for their profile.
+4. The user performs a Desmos transaction telling that they want to link the Instagram account to the Desmos one, and provides the proper call data.
+
+Once that's done, what will happen is the following:
+
+1. Desmos will send the call data to Band Protocol, asking to get the Instagram user username and the signature provided by the user.
+2. Band Protocol will call the appropriate data source that will use our APIs to get the data from the account biography.
+3. Once downloaded, the data source will check the validity of the data and return to Desmos the user username and signature.
+4. If the Instagram user username matches the one provided by the user, and the signature is valid against the user public key, the Desmos and Instagram account will be linked together.  
+
+## Data source call data
+
+When asking to verify the ownership of a Instagram account, the data source call data must be a JSON object formed as follows:
+
+```json
+{
+  "username": "<Instagram user username to be verified>"
+}
+```
+
+Example:
+
+```json
+{
+  "username":"test_desmos_user"
+}
+```
+
+Hex encoded:
+```
+7b22757365726e616d65223a22746573745f6465736d6f735f75736572227d
+```
+
+Example execution:
+
+```shell
+python Instagram.py 7b22757365726e616d65223a22746573745f6465736d6f735f75736572227d
+```