doc: move Node 8.x to Active LTS

Also changes `Active` and `Maintenance` to `Active LTS` and
`Maintenance LTS`.

Fixes: nodejs/Release#257
Refs: nodejs/node#16630

Author: Soufiane88216

README.md

@@ -2,17 +2,17 @@
 
 ## Release schedule
 
-| Release |  LTS Status   | Codename | Active LTS Start | Maintenance Start | Maintenance End |
-|   :--:  |    :---:      |   :---:  |       :---:      |       :---:       |      :---:      |
-|  v0.10.x  |**End-of-Life**|    -     |        -         |    2015-10-01     |   2016-10-31    |
-|  v0.12.x  |**End-of-Life**|    -     |        -         |    2016-04-01     |   2016-12-31    |
-| [4.x][] |**Maintenance**| [Argon][]|    2015-10-01    |    2017-04-01     |   April 2018    |
-|  5.x    |No LTS         |          |                  |                   |                 |
-| [6.x][] |**Active**     | [Boron][]|    2016-10-18    |    April 2018     |   April 2019    |
-|  7.x    |No LTS         |          |                  |                   |                 |
-|  8.x    |**Pending**    | Carbon  |    2017-10-31  |    April 2019     |   December 2019<sup>1</sup> |
-|  9.x    |No LTS         |          |                  |                   |                 |
-| 10.x    |**Pending**    | Pending  |    October 2018  |    April 2020     |   April 2021    |
+| Release | LTS Status          | Codename  | Active LTS Start | Maintenance LTS Start | Maintenance LTS End       |
+| :--:    | :---:               | :---:     | :---:            | :---:                 | :---:                     |
+| v0.10.x | **End-of-Life**     | -         | -                | 2015-10-01            | 2016-10-31                |
+| v0.12.x | **End-of-Life**     | -         | -                | 2016-04-01            | 2016-12-31                |
+| [4.x][] | **Maintenance LTS** | [Argon][] | 2015-10-01       | 2017-04-01            | April 2018                |
+| 5.x     | No LTS              |           |                  |                       |                           |
+| [6.x][] | **Active LTS**      | [Boron][] | 2016-10-18       | April 2018            | April 2019                |
+| 7.x     | No LTS              |           |                  |                       |                           |
+| 8.x     | **Active LTS**      | Carbon    | 2017-10-31       | April 2019            | December 2019<sup>1</sup> |
+| 9.x     | No LTS              |           |                  |                       |                           |
+| 10.x    | **Pending**         | Pending   | October 2018     | April 2020            | April 2021                |
 
 Dates are subject to change.