Add simple README.

Author: bbockelm

README.md

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+
+SciTokens C++ Library
+=====================
+
+This repository implements a minimal library for creating and using SciTokens from C or C++.
+
+[SciTokens](https://scitokens.org) provide a token format for distributed authorization.  The
+tokens are self-describing, can be verified in a distributed fashion (no need to contact the
+issuer to determine if the token is valid).  This is convenient for a federated environment where
+several otherwise-independent storage endpoints want to delegate trust for an issuer for
+managing a storage allocation.
+
+Building
+--------
+
+To build the `scitokens-cpp` library, the following dependencies are needed:
+
+   - [jwt-cpp](https://github.com/Thalhammer/jwt-cpp): A header-only C++ library for manipulating
+     JWTs.
+   - OpenSSL 1.0 or later.
+   - `sqlite3`
+
+CMake is used for the build system.  To build, from the source directory:
+
+```
+mkdir build
+cd build
+JWT_CPP_DIR=~/path/to/jwt-cpp cmake ..
+make
+```
+
+Testing
+-------
+
+The easiest way to test `scitokens-cpp` is to head to the [SciTokens Demo app](https://demo.scitokens.org)
+and copy the generated token.  Then, from the build directory:
+
+```
+./scitokens-verify  eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsImtpZCI6ImtleS1yczI1NiJ9.eyJpc3MiOiJodHRwczovL2RlbW8uc2NpdG9rZW5zLm9yZyIsImV4cCI6MTU0NjQ1NjMwOSwiaWF0IjoxNTQ2NDU1NzA5LCJuYmYiOjE1NDY0NTU3MDksImp0aSI6ImRlYmNkZDRjLTU1MzgtNDkxNS1hY2U2LTgyNTg3NGQwZjEzNyJ9.Vu9TRfDi5WJujeAGl-wP-atvNqh31-gteKqqu_IEcxoCfGYdmoIM3xOOY1GmHcmXfclkrl724ldxBBChsDpcdi_8914N9EGwGVApJLQU0SaPPdtcoCrqvVJE3bD9fs6UKooGwuk_e20ml9g0R4100fTdsD7pkIOABYGTbhxioEb1dP1o-17l2t2kUXpd8KhIyZZjmtmnMdmO5bxaY_V60OOWekwelT8ACK8ao39Ocf_wmUiS0VVX21hD1KqO0bgBU9AsVJ5prAL9ytElr_UB2X5KowPODbj6LPFNhpCwXcoG4w4Gw9VueuxCuIPhlcHBhP83i5LPgtk2YOjygdSahA
+```
+
+Replace the given token above with the fresh one you just generated; using the above token should give an expired
+token error.
+