docs: add mempool info

Fixes blockstack/docs#1044

src/pages/understand-stacks/technical-specs.md

@@ -62,6 +62,7 @@ description: Summary of technical specifications of Stacks 2.0
   3. _sending account_ is the account that identifies who is currently executing the transaction: this can change as a transaction executes via the `as-contract` Clarity function
 - Two types of authorizations: standard authorization is where originating account is the same as paying account. _Sponsored_ authorization is where originating account and paying account are distinct. For instance, developers or service providers could pay for users to call their smart-contracts.
 - For sponsored authorization, first a user signs with the originating account and then a sponsor signs with the paying account.
+- Mempool limit for concurrent pending transactions is 25 per account
 - Transaction encoding is described [here](https://github.com/blockstack/stacks-blockchain/blob/master/sip/sip-005-blocks-and-transactions.md#transaction-encoding) and [here](/understand-stacks/transactions#encoding)
 - Transaction signing and verification are described [here](https://github.com/blockstack/stacks-blockchain/blob/master/sip/sip-005-blocks-and-transactions.md#transaction-signing-and-verifying) and [here](/understand-stacks/transactions#signature-and-verification)
 - All transactions impacting account balance are atomic, a transfer operation can not increment one account’s balance without decrementing another’s. However, transactions that perform multiple account actions (for example, transferring from multiple accounts) may partially complete.