doc: Add LICENSE

LICENSE

+<SysCore Is A OpenSource Operating System for RiscV, Able To Run On Kendryte-K210 Development Board>
+Copyright (C) <2021>  <iPear@https://github.com/i-Pear, Waitti@https://github.com/Waitti-long, fluctlight001@https://github.com/fluctlight001>
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
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