Adding upgrade Indexer to glossary (#507)

* Adding definition

* Update website/pages/en/glossary.mdx

Co-authored-by: Benoît Rouleau <benoit.rouleau@icloud.com>

* Update website/pages/en/glossary.mdx

Co-authored-by: Benoît Rouleau <benoit.rouleau@icloud.com>

---------

Co-authored-by: Benoît Rouleau <benoit.rouleau@icloud.com>

Author: MichaelMacaulay

website/pages/en/glossary.mdx

@@ -24,6 +24,8 @@ title: Glossary
 
 - **Indexer's Self Stake**: The amount of GRT that Indexers stake to participate in the decentralized network. The minimum is 100,000 GRT, and there is no upper limit.
 
+- **Upgrade Indexer**: A temporary Indexer designed to act as a fallback for subgraph queries not serviced by other Indexers on the network. It ensures a seamless transition for subgraphs upgrading from the hosted service by readily serving their queries upon being published. The upgrade Indexer is not competitive with other Indexers, and supports chains that were previously exclusive to the hosted service.
+
 - **Delegators**: Network participants who own GRT and delegate their GRT to Indexers. This allows Indexers to increase their stake in subgraphs on the network. In return, Delegators receive a portion of the Indexing Rewards that Indexers receive for processing subgraphs.
 
 - **Delegation Tax**: A 0.5% fee paid by Delegators when they delegate GRT to Indexers. The GRT used to pay the fee is burned.