Regroup all Exclusive keyboard in one directory

To reduce clutter in the `keyboards/` directory, this patch moves all
keyboards definitions for Exclusive boards to its own subdirectory.

keyboards/exclusive/e6_rgb/readme.md

@@ -7,6 +7,6 @@ Hardware Supported: E6V1, E6V2
 
 Make example for this keyboard (after setting up your build environment):
 
-    make e6_rgb:60_ansi
+    make exclusive/e6_rgb:60_ansi
 
 See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).

keyboards/exclusive/e6v2/le/readme.md

@@ -10,10 +10,10 @@ Hardware Availability: [geekhack.org/index.php?topic=90787.0](https://geekhack.o
 
 Make example for this keyboard (after setting up your build environment):
 
-    make e6v2/le:default
+    make exclusive/e6v2/le:default
 
 Or to make and flash:
 
-    make e6v2/le:default:dfu
+    make exclusive/e6v2/le:default:dfu
 
 See [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) then the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information.

keyboards/exclusive/e7v1/readme.md

@@ -10,6 +10,6 @@ Hardware Availability: [https://geekhack.org/index.php?topic=97182.msg2654226#ms
 
 Make example for this keyboard (after setting up your build environment):
 
-    make e7v1:default
+    make exclusive/e7v1:default
 
 See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).

keyboards/exclusive/readme.md

@@ -0,0 +1,9 @@
+# Exclusive Firmware
+
+All files related to firmware of keyboards designed by Exclusive.
+
+## Keyboards
+
+* [e6_rgb](./e6_rgb/): maintainer [astro](https://github.com/yulei)
+* [e6-v2](./e6v2/): maintainers [MechMerlin](https://github.com/mechmerlin), [amnesia0287](https://github.com/amnesia0287)
+* [e7-v1](./e7v1): maintainer [masterzen](https://github.com/masterzen)