doc: add document about tctis

Signed-off-by: Johannes Holland <joh.ho@gmx.de>

README.md

@@ -21,7 +21,7 @@ This stack consists of the following layers from top to bottom:
 | Enhanced System API (ESAPI,&nbsp;sometimes&nbsp;ESYS) | libtss2&#x2011;esys | 1-to-1 mapping of the TPM2 commands<ul><li> Session handling</li><li>Tracks meta data for TPM objects</li><li>Asynchronous calls</li></ul> | [TCG TSS 2.0 Enhanced System API (ESAPI) Specification](https://trustedcomputinggroup.org/wp-content/uploads/TSS_ESAPI_v1p0_r08_pub.pdf) |
 | System API (SAPI,&nbsp;sometimes&nbsp;SYS) | libtss2&#x2011;sys | 1-to-1 mapping of the TPM2 commands<ul><li>Asynchronous calls</li></ul> | [TCG TSS 2.0 System Level API (SAPI) Specification](https://trustedcomputinggroup.org/wp-content/uploads/TSS_SAPI_v1p1_r29_pub_20190806.pdf) |
 | Marshaling/Unmarshaling (MU) | libtss2&#x2011;mu | (Un)marshaling all data types in the TPM library specification | [TCG TSS 2.0 Marshaling/Unmarshaling API Specification](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TSS_Marshaling_Unmarshaling_API_v1p0_r07_pub.pdf) |
-| TPM Command Transmission Interface (TCTI) | libtss2&#x2011;tcti&#x2011;device<br>libtss2&#x2011;tcti&#x2011;tbs<br> libtss2&#x2011;tctildr<br>libtss2&#x2011;tcti&#x2011;swtpm<br>&#8230; | Standard API to transmit/receive TPM commands and responses | [TCG TSS 2.0 TPM Command Transmission Interface (TCTI) API Specification](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TSS_TCTI_v1p0_r18_pub.pdf) |
+| TPM Command Transmission Interface (TCTI) | libtss2&#x2011;tcti&#x2011;device<br>libtss2&#x2011;tcti&#x2011;tbs<br> libtss2&#x2011;tctildr<br>libtss2&#x2011;tcti&#x2011;swtpm<br>&#8230; | Standard API to transmit/receive TPM commands and responses<br><br>See [doc/tcti.md](doc/tcti.md) | [TCG TSS 2.0 TPM Command Transmission Interface (TCTI) API Specification](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TSS_TCTI_v1p0_r18_pub.pdf) |
 ||| Basis for all implementations in this project. [1] | [TCG TSS 2.0 Overview and Common Structures Specification](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TSS_Overview_Common_Structures_v0.9_r03_published.pdf) |
 
 [1] We currently deviate from the specification by increasing the value of `TPM2_NUM_PCR_BANKS` from 3 to 16 to ensure compatibility with TPM2 implementations that have enabled a larger than typical number of PCR banks. This is expected to be included in a future revision of the specification.