[MAINTAINERS] New maintainer candidate: Karim Taam (#640)

* I propose to add Karim as a Besu project maintainer.

Karim contributed with many high quality commits (including bounties):
- NAT,
- log filters,
- fast sync,
- tracing.

Here are [his past contributions on Besu project]
(https://github.com/hyperledger/besu/commits?author=matkt)
and he even did some before the client was renamed to Besu.

Voting ends two weeks from today at
[midnight UTC on 2020-04-17](https://bit.ly/2JzWqd4)

For more information on this process see the
Becoming a Maintainer section in the MAINTAINERS.md file.

Signed-off-by: Nicolas MASSART <nicolas.massart@consensys.net>

MAINTAINERS.md

@@ -20,6 +20,7 @@
 | Sally MacFarlane | macfarla         | macfarla         |
 | Madeline Murray  | MadelineMurray   | madelinemurray   |
 | Mark Terry       | mark-terry       | m.terry          |
+| Karim Taam       | matkt            | matkt            |
 | Meredith Baxter  | mbaxter          | mbaxter          |
 | Nicolas Massart  | NicolasMassart   | NicolasMassart   |
 | Stefan Pingel    | pinges           | pinges           |
@@ -90,4 +91,4 @@ maintainer to emeritus status is recorded in the github PR making that change.
 
 Returning to active status from emeritus status uses the same steps as adding a
 new maintainer. Note that the emeritus maintainer already has the 5 required
-significant changes as there is no contribution time horizon for those.
+significant changes as there is no contribution time horizon for those.